Поздравления с днем рождения на открытке на английском языке в информальном стиле

Oh, you grow up very fast! Every year you become older and older! And how old are you now? 15 or 16? (можно другие цифры) You are so old person! But I still respect and love you (Of course, only like a friend). Happy birthday! I wish you don't grow up so fast, and get only the best marks!

With love, your best friend.;)