24+ years flying, 10,000+hrs and now in the LH seat of a BIG jet - but I am still instructing on days off !!
When you look at what you are doing and decide 'do I still want to be doing this in 20 years time', then is the time to move on, but you don't have to give up the flying you already enjoy.
Have fun (the big machines CAN be fun to).
