I also don't get it.

I'm 21.

I remember getting it twice in the 90s (1996 and 1998) - in memory, I've probably vomited and/or had it when I was a baby/toddler in the early 90s.

Twice again in the 2000s - very briefly in 2001/2002 (dunno which year exactly) and the MOST BIGGEST, WEIGHT LOSING, LIFE DRAINING, LIFE SUCKING, DEPRESSING, LONGEST ONE was in July 2008 - oh God it ruined my life. It went for 2 weeks and I didn't vomit it at all (lucky that we emets have this technique of holding it back). I lost 8 kilos and now I'm still underweight. It was my personal '9/11'.

Since it seems like I don't have the stomach flu much I believe yes, some of us are not prone to it as others are. It depends on our blood type I guess.