If you have big when you're young will they be saggy when you are older?

Right now I am 17 with 36D and sometimes a DD....they are perky and all but thats because I'm only 17. When I get older though are they destined to drop? I mean, I know when I'm an old woman my s are going to be saggy...but I mean like in my 30s and 40s....thanks for the answers!