Great question, and would think the answers vary a whole bunch.
Started in 1963, never stopped or paused unless recovering from injuries (thank you two decades plus of high country snowboarding) and would view it as:
Peak was in my mid 20’s thru my late 30’s. Boards averaged 6’4x 19.5 (I’m 6’3 x 185)
Slow performance drop from 40 to 50, offset with a bit more board and the board themselves just working better (boards creeping up to 7’6 + x 20 for Norcal waters), more focus on down the line from deep rather then vertical, increased committment to daily fitness regiment, and making sure I was still putting in 100 to 150+ days of surfing annually regardless…
50 to 60…now there’s where the drop takes place, no way around it. The key at this stage is to keep surfing, continue to keep your weight optimized (rare to see a ripping over weight older surfer), your fitness workouts ongoing, and very importantly tweak your equpment so you never find yourself struggling to be competative in the peak. Current quiver starts at 7’8 x 21 and goes up to an 8’6 HP epoxy longboard for smaller summer. Well designed boards work so well these days a bigger board doesn’t mean
less fun for non-cruzers still wanting to rail it and bash a lip…Still trying to get in 75 days a year (winters in the Pacific NW are fairly brutal). Double sessions are fairly rare, a good two - thre hour session and usually pau for the day.
Most important thing during the journey - never stop surfing, focus on the joy of just paddling out and feeling the waves break over your head, just grateful to still be here, to still be dancing on the water…