Some experts will tell you that you should aim to be the weight you were when you graduated from high school, around age 18. (Only exception is if you were severely overweight.) Also the reasoning here is that we tend to peak around age 18-21 in terms of lean mass - particularly bone. (There is never an easier time to gain lean mass than in your teens, which is why it is not recommended for teens to limit calories... but I digress... ).
Even if a little overweight at that age, these health expert will tell you that those who managed to maintain their weight at age 18 (+/-5 lbs) are healthier as adults and live longer. Wildly fluctuating weight throughout adulthood, is more unhealthy than achieving a particular "ideal" weight. And by maintaining your teenage weight, you are effectively maintaining your peak lean mass. (Of course there are exceptions... you can build muscle mass anytime, it's just harder as an adult.)
I think 18 is a bit restrictive. I'd pick age 21 instead. Because when I was 18, I weighed 103, but I hadn't yet reached my full adult height, and I don't think that is uncommon at all. I grew into my whopping 5'0" sometime in college. So instead, I go by my college graduation weight - 107 lbs. Then I pick a range, +/- 5 lbs from that weight. Because it's normal to fluctuate and it'll drive you batty trying to stay at an exact number.
Through experience I know that if I drop too much below 107, I feel weak and sickly. I can be happy and like the way I look with my weight up until about 115. Once I go over 115, my blood sugar tends to go all whacky on me, and I get horrible heartburn. And clothes start to fit not so well.
