birthday parties

why do we stop having birthday parties? when do we hit an age where some one says i'm not having you a party anymore because you are to old to have parties? why is it that every year of our life isn't worth celebrating with all our friends & family? when you become married you generally just do a dinner/movie type thing with your spouse for your birthday or they may even surprise you by taking you away as a gift. but why can't we do those things as well as celebrate with a big bash with everyone we love? i mean you survived yet another year of life so why not have fun & have a party because of it?