ill be 30 in a month and i still get asked for I.D. i look in my early 20s, if i have had a shave i look even younger. Theres that saying that time goes fast if you're having fun. i try and enjoy life always and never let things stress me out. any "negative" i try and take a positive from it, learn and empathise etc.

im also a vegetarian and look after myself. like the OP i recently met up with a few old school friends and they looked worn out, old fat and unhealthy.

sunlight, fresh air, excersise, diet and mental attitude and spiritual perception i believe are key components to "younger" appearance.

what is also interesting is that when i was younger i used to watch some jerry springer and noticed a lot of the teenagers looked a lot older than usual. later as a grew up and looked into how diet can affect puberty, hormones and a whole bunch of biomechanics factors, the old saying you are what you eat never felt so powerful.
so i don't necessarily think its that some people look young, but perhaps people are ageing at a quicker rate due to the impact of their environment. holistically, if you are healthy, happy and living in the moment, then there is no reason not to stay looking young.