I don’t think discussing those topics early on is automatically a red flag - it really depends on how the topic is discussed.

It’s generally a good idea to talk about what your goals and wants are in a relationship. ‘Success’ in a relationship doesn’t look the same to everyone, and it’s crap to end up in something serious with someone who has a different end-goal than you. If someone says ‘ultimately I’d like to get married one day’ that’s probably just them communicating with you.

If someone starts telling you that they’re positive that they definitely want to marry you after a short time, that’s when it’s likelier to be indicative of some pretty negative things.

Your girlfriend sounds like she’s telling you that A) marriage is a relationship goal for her and B) so far you fit her criteria as someone she could meet that goal with. It doesn’t sound like a proposal. Bear in mind though, that at 16/17, everyone is usually still figuring themselves out and learning what they actually want in relationships. People try on ideas a lot at that age, so not everything ends up meaning something significant.