27 year old Junior right here. I went back when I was 25. It's almost a total non-issue. In some cases I find I work a little harder or take things a little more seriously than some of my peers, but even that isn't as significant as I thought it would be. Actually, there's a fair number of people around my age in the program. A lot of them are military guys too. I don't think you'll have a problem blending in.
There's a lot of great advice in this thread, but I want to focus on something that I haven't noticed anyone saying. Going back to school, especially for a demanding program like engineering, is tough on your adult relationships. I spend 8 or 9 hours a day at work or school, and another 2-4 hours studying or doing homework. At least one weekend day ends up being mostly a study day as well. It was tough for my girlfriend to adjust to seeing me less, and we had some fights about it at first. Few things feel shittier than getting into it with the person who is closest to you when you're so exhausted from solving integrals that you can't see straight. I'd recommend that you and your wife discuss the time commitment. I always try to remember that while I know my girlfriend is behind me, she didn't sign up for this shit, and I owe it to her to keep my studies from impacting our relationship.
It's also tough to stay connected with friends that aren't in school. If I have trouble seeing my girlfriend sometimes , whom I live with, you can imagine how hard it can be to drag my ass out for a drink during finals week. Try to make the time. As school goes on, you'll start to get a feel for when you need to put down the books to avoid burnout. Call up a friend and go play football, or video games, or drink, or whatever it is you do for fun. You probably won't make a ton of new friends at school, so make sure you're keeping the ones that you've already got. And when you are out with your friends, remember: They're not in college anymore! They don't give a shit how hard dynamics is! Talk about something else and save that for your friends at school.
All in all, going back to school was one of the best decisions of my life. I'm learning so much, and I am on a clear path to where I want to go in life. It's not easy, but it's right. I hope you find your experience to be just as positive.
tl;dr: do it! but know that it's a heavy time commitment, and that you will need to go out of your way to stay connected to the people in your life who already care about you.