From my understanding, it’s a generally MAP-positive holiday that was founded to celebrate the life of Alice Liddell (the inspiration for “Alice in Wonderland”) and the bond between her and Lewis Carroll. I believe there’s more to it overall, but I’ve always tended to simply celebrate the day as a symbol of connection between MAPs and our workings to create a more accepting society for us (and all paraphiles).
Welcome! There’s nothing to be ashamed about for seeking out relationships with older people, just as I don’t think there’s anything wrong with seeking relationships with younger people. I’m sorry that you were told you’re getting “too old” by someone, but I guarantee there are plenty of people who think 15 is a very desirable age.
Putting all that aside, the main key is that you should be happy with yourself and who you are, and if you’re going to be in a relationship, make sure that they care for you the same way you care for them. If it’s just a relationship based on passion that you both don’t plan to keep going forever, that’s fine, but make sure you both communicate that with each other. If you want the relationship to be more romantic and last much longer, make sure both of you are on board with that, too.
I hope we can all find relationships like that.