This is a common problem. The startup has "grown above" you and it's time to consider moving on, because unless you start playing politics you're going to get demoted with each new hire. Negotiate a better title and salary (you'll probably get something) so you can get a better next job, and see about getting investor contact (preferably of a social nature rather than "presenting to" investors, because you want social equality in order to get investor-level mentorship that persists even if they can't fund your next bit) but you'll probably be out the door in another 6 months.

Don't take it personally. Even if it is personal, it's not worth getting bitter about it. The pro-young ageism only works for Stanford kids; if you're an average 24-year-old, this game still treats you like shit in most cases. The "social climbing" dynamic in startups isn't uncommon. It sounds like they don't have a culture of internal promotion. This will burn them, later on, because people will catch on to the social-climbing/no-internal-promotion culture and everyone will start doing bare-minimum work... but you probably won't be there when it does.

Ask for a VP-level title, even if it doesn't involve you managing anyone. If you don't have the experience to be the VP of Engineering, then you can't expect them to give you that title, but you should get something at the same level, so that you have political equality with these new guys. Even if it's a meaningless VP title ("VP of Technical Culture") it at least shows that they're committed to giving you the credibility that you need to continue contributing meaningfully to the company. No one's going to take you seriously if you're Employee #2 and don't have a title, and you can make this argument in the negotiation. And saying "no one's going to take me seriously if I don't get <X>, and I won't be able to do my job" is a great way of saying "I'll leave if I don't get <X>" without actually saying it; it has the added benefit of speaking from a perspective of their needs (your ability to do your job) rather than yours.

If you want to be a founder or CTO in your next gig, demand investor contact. If they won't give it, the words you want are, "If you're not willing to give me this, the right thing to do is to accelerate my equity vesting, so we can separate cleanly." You're not actually threatening to quit, but you're strongly implying it in a way that (a) makes it clear that your needs aren't going to go away, because you've started to think seriously about the future, but (b) doesn't commit just yet to one course of action. (They could re-up your equity, or give you the investor contact, in which case you'd stay.)

If your founders are good and this just happened "accidentally" or because you had your head down and didn't fight for yourself, you can improve your position significantly. You may not get everything you want (you might have to settle for a Director title and investor contact only to the more junior partners) but you'll make progress and be ahead of most people your age. On the other hand, if they're deliberately fucking you up the ass, then expect them to denigrate your work. Again, don't take that personally either; they're taking a political decision (to grow above and demote you, because they're social climbers) and back-rationalizing it. So don't let that, should that happen, embitter you either; just move on.

Good luck!