I get the "just wait until you get older and become fat" line all the fucking time. For the longest time, I believed it, because I never really thought much about it.  Sometimes, I'll respond by telling them that both my parents (who are in their 50s) are thin like me, so I don't think age is going to cause me to suddenly turn into a fat fuck. Of course, 99% of the time, their response is "Oh, you're so lucky, it sounds like you have really good genes. I hate you." It's called self control, you fat fucking fucks.