I got my first tattoo in my mid-30s, and I spend 3 years thinking about it -- not just the what, and whether or not I actually wanted it, but the where. There are many places on the body that don't suffer the effects of gravity and weight gain/loss, and I specifically chose one of those (the very top of my arm) for that very reason. The other reason I chose that location is that it allows me to control when the tattoo will be visible to others just by wearing or not wearing sleeves.
Then last year I got my second one -- to memorialize my beloved kitteh -- in the same place on the other arm. I don't plan to get any more, though I might add details to the two I have.
I think getting a tattoo in your teens or early 20s is silly not just because of the aging & sagging issue, but because it's pretty rare to know at that age what kind of symbol/image you're going to want with you for the rest of your life.