After some time after weight training is over, does muscle mass really disappear?

[darkshadowofmordor]

usually as a boy without any weight training, one's muscles aren't large and stronger. i've noticed that while i'm weight training, i see muscle gain. but after a month after stopping, it started to go away. yet i see men, my dad and others who don't do any sort of weight training anymore and haven't for many years yet their arms for example, look reasonably good and not like a young boy's. why is this? how can i make muscle last permanently like theirs?