I’m over 50 and do quite a lot in the gym to stay fit for biking and skiing.
I cycle twice a week usually, and go to the gym between 5 or six times. Some work in the gym is on the stationary bike, some on the stepper and the cross country thing, but mostly it’s free weights.
The free weight thing varies between the usual back/chest, shoulder, arms etc, but mostly it’s a circuit type session with kettle bells, tyres, sledges etc.
I also make sure I stretch quite a lot, especially back and legs.
I’m pretty sure it helps with the bike; certainly my arms never feel “pumped” after big descents. I work on the basis that any fitness work is better than none.