How to stay slim when old?

Hello there. This question is for the older people. I know I shouldn't be concerned right now, but I worry a lot about weight gain and being lazy when older. Right now, I am still loosing weight and once I am at my goal, I would like to stay that way forever (that's a big goal in my life). Ladies and gentlemen, how do you stay slim and healthy and especially after menopause (women)? What are some of the foods, beverages and desserts I shouldn't consume? I actually am a vegan, but even so, I still worry about it. I am now age 16, 5"6", 152.9 Ibs and exercise 6 times a week on my elliptical (1hr, sometimes 2 hours). Not only am I a vegan, but I don't consume rice, pasta, bread (very, very rare I have bread), peanuts (am not allergic), alcohol,margarine, butter, refined sugar and I do not consume fried foods anymore (probably do not eat a lot of other bad foods anymore like ice-cream). Please respond and I appreciate the answers that are given, Thanks!!!! :D