When I was in university there were quite a few "older" students. We even had grandparents! Before I enrolled at my university I took some classes at a community college and almost half of the students in my class were older people who were changing careers. One guy in his 40s had worked as an engineer for 20+ years and said he finally "got fed up with the bullshit" and went back to school to become a physicians assistant. None of us younger students judged or looked down on the older students at all. As far as we were concerned they were one of us and we were all suffering together! You may feel like you stand out, but aside from the first day when people are checking out who's in their class no one will give a shit because everyone is too worried about themselves and their grades. So no, you're definitely not too old :)
Student clearinghouse reported that between 2009 and 2011 over 1/3 of students were over 25. Here's the PDF of the report. This article says 73% of students are "nontraditional" - many of them older adults returning for a career change. So it looks like you're not alone.
Look for nontraditional, returning student, or older student services and groups on campus just so you can meet other people in the same boat and so you have support on campus.
Also, I screwed around a lot during my undergrad and made a way too many mistakes. I'll give you some advice so you can hopefully avoid all the mistakes I made.
1) Buy used textbooks online. I didn't learn about that until my junior year - I could have saved myself hundreds and maybe even thousands of dollars that I spent on overpriced textbooks. Other people will tell you to torrent them, but if you don't want to do that there are plenty of used textbook sites or even amazon. Just make sure you comparison shop!
2) Study guides and study groups are your best friend. Most of my classes handed out study guides before big exams with really detailed questions. Every time I did the study guides I aced the test. Make friends in class. Working in study groups is a good way to have confusing material explained to you from a new perspective or a good way to commit knowledge to memory if you have to explain it to a peer. Study groups are also a really helpful way to work on study guides and make sure you didn't miss anything. Before tests review the study guides alone and in a study group.
3)Most universities and community colleges offer some kind of tutoring service - take advantage of these.
4)Your professors and TAs will have office hours. I was insanely shy and too scared to ask for help when I needed it and it hurt my grades. During my senior year I stopped being such a coward and realized that more often than not my professors were really happy to help me. Some even just liked chatting and get to know their students. There were a couple of assholes that seemed to hate their jobs, but don't let those deter you because the majority are nice people who want you to succeed.
5)Take good notes in class and then review them and highlight the most important information. Sometimes the professors will post the PowerPoints of their lectures online. The first time I encountered this I thought it was a good way to slack off. Big mistake. Physically taking notes helps commit the information discussed in class to memory.
6)Flashcards are insanely helpful for committing to memory information that just won't stick no matter how many times you review the material. If you'd prefer to use online flashcards there are a lot of programs available online. I personally like Quizlet. Quizlet let's you share your flashcards with groups so it's also a good tool to use for study groups. You also have the option to combine flashcards with other people.
7)If you absolutely fail a class don't stress. You can always retake the class and whichever class has the highest grade will replace the one with the lowest. It's expensive and time consuming, but if you're concerned about your grades just know it's an option.
8)If you have any kind of testing anxiety universities offer special testing services.
9)Someone already mentioned this, but it's so important I'm going to mention it again. Internships are so helpful in future job placement. I completely neglected this and screwed myself, whereas all of my friends who took advantage of internships found jobs right after college. It also helps if you talk to your professors because they can offer guidance, maybe steer you in a better direction career wise that you otherwise wouldn't have known about, and let you know if they are personally looking for student help or interns. My ex used to talk to one of our professors after class everyday and out of nowhere he offered him a student research position in our department. He didn't have the best grade in our class, he wasn't the smartest in our class, my professor simply just liked him and liked that he showed interest in the field.
10)Someone already mentioned this too, but if you can try to do your prereqs at a community college. From my personal experience, community colleges are much better at helping students adapt to college life. It's smaller, there are a lot of learning resources, there are a lot of nontraditional students, and you still have many of the same benefits you'd have at a university. My bio professor at my CC taught the exact same course I was taking at the university for like 3X the cost of what I was paying. The biggest advantage was we always had him for office hours whereas the university students mostly had the TAs. I also had an amazing TA at my cc who once stayed after class for over 3 hours to help me with some math I was struggling with.
11)Network. Network with your professors, TAs, other students, people in your department, places you intern, everyone. I fail miserably at this because I'm so shy and it hinders me in a lot of ways.
12)This may sound like overbearing motherly advice, but seriously eat breakfast and maintain a regular sleep schedule. You won't be able to concentrate in hour long lectures all day long if you're not eating properly and getting enough sleep. Also, if you're going to be on campus all day take water and snacks. I used to take fruit, mixed nuts, granola bars, sandwiches, etc. Make sure you get enough protein too because otherwise your energy will crash.
13)CC's and universities offer new student seminars and tours to get students acquainted with the campus and all the services. I didn't do this and I was lost for like the first month of school.
14)Take advantage of all the random symposiums and seminars on campus. My university held the Darwinfest which was a series of scientific events and symposiums for 4 months. Researchers from all different fields gave amazing lectures. I went to an astrophysics lecture at a planetarium that was so awesome I don't even have the words to explain it. It feels amazing to be surrounded by people who want to create a better world and who share your interests.
15)The final point is going to sound cliche and stupid, but it's important. You're going back to school for yourself. It's your life and you're doing it because it's important to you. Don't compare yourself to other people or let others drag you down. You don't need their approval. Do what you need to do to succeed and be happy. If you make mistakes realize that it's ok and take advantage of all the services I mentioned to you. Most people in your classes are going to be as lost and confused as you might feel. It's easy to get bogged down or depressed due to all the stress in college, but remember you're not alone and remember why you're doing it. If you keep your goal in mind and don't give up you'll be fine!
I'm really tired so I hope what I wrote is at least somewhat coherent! If you have any other questions feel free to ask me even if you're worried it will sound stupid. I had no one to talk to when I started college and I was completely overwhelmed and had no idea how anything worked - not financial aid, not parking passes, not the textbook system, nothing! Anyway, you have people here willing to answer any questions you have so take advantage of it.