I’ve done both Topdeck (2, 3rd next year) and Contiki (~10). I’ve been one of the oldest on all of the trips I’ve done so more or less the opposite of you. I haven’t really had an issue with the age ranges, you’re all there for the travel experience. On one of my trips 4 years ago I met a girl at the younger end of the age range that I got along great with, we still keep in touch, even met up again last year (I’m in New Zealand, she’s in Canada, the far side from NZ). I’ve had smallish groups (smallest was 11, ~20 is a good number) up to full large buses (~50) and haven’t had that much difficultly finding someone to explore with (I’m shy, not confident when by myself).
Can’t talk to so much to the fun part as I’m not much fun, but looking back there was a trend for more nights out drinking on the Contikis. As I don’t drink and can’t handle loud noisy places I didn’t participate in that often so didn’t really notice at the time. I’ve also only done 2 Topdecks so far so may have just been the tours I’ve done.
I’m sure you’ll have a great time no matter which one you decide to take. Just pick the one that fits your timetable best and ticks the most travel goal boxes.