First let me say congrats on your comeback to the sport, and for sticking with it for 4 years. I came back in my mid-40's after a 27 year layoff and know how hard that was. I can only imagine how much determination it must have taken at the beginning to stick with it.
I'd say if you can do a 25 fly at all, you are a stronger swimmer than 99% of people your age. Doing it in 20 seconds puts you in the 99%+ category!
USMS doesn't keep stats for 25 fly (even though it is occasionally offered at USMS meets), but they do for the 50 fly. Last season, 56 men in the 60-64 age group recorded a 50 yard fly time under 30 seconds. 195 recorded a 50 fly time in under 40 seconds. So, if you could hold your pace for a 50, you'd be in the top 200 in the country in your age group. That wouldn't be too shabby for somebody that took a 40 year hiatus!
After focusing on freestyle for my first year of my comeback, I've added butterfly to my repertoire this season. I'll be swimming the 50 fly at the Illinois Masters State Meet next weekend. I got a lot out of this podcast:
(starting at episode 6).
Dropped a second/25 off my pace in the last 6 weeks by following their guidance, particularly with regards to getting the head back down early in the stroke after breathing, minimizing the porpoising (i.e. keeping flat), and keeping the hand entry slightly wider than shoulder width.