You should really think about how the things you say will affect another person. I'm sure you think you're being complimenting by telling someone they look so young, or by telling them they should feel grateful, they will feel that way. Sorry to burst your bubble, but we don't. It's not complimenting, and it's not something I'm going to feel grateful for right now! Maybe sometime in the future, yeah, when I don't look my age, but right now, it's the last thing I want to hear.
I'm sure there are those out there that will disagree with me, or try to convince me of the opposite, but being a person who deals with it on a daily basis, I can tell you how unpleasant it is! I can't even count the number of times I've been mistaken for being under 15. I'm 21. I've been carded for video games and PG-13 movies! Buying alcohol is a nightmare. I'm always worried that they're going to yank my ID, accusing me of handing over a fake. Not only that but shopping in stores I've been told, "Oh don't worry. You'll grow into them." when I've asked for a smaller size.
I'm much more careful in my interactions with men as well. Lately when I've been approached, I've been told I look 14, and then they continue on to ask for my number. If that's not creepy, I don't know what is. "Hey you look like a little girl, want to date?"
I can't easily change the way I look, no matter how frustrated it makes me, but other people most certainly can change the way they talk to me and the things they say. So please consider this, and think before you speak! This can apply to anything, not just remarking about how old somebody looks, so please be considerate of others! You never know if what you're saying is going to be taken as a compliment, or if it will just beat down someone's self-esteem even more!