The most convenient age - 25
I think it depends. Some people are happy teaching all their lives and will "retire" at whatever age they become pensionable in their own particular country. And stop. or maybe not - they may go on giving a few private lessons etc etc. But I think a lot of people have reached burn-out by then. Especially those in the State system, and those who have never "diluted" the number of hours they teach with other related work : teacher training, ELT management, materials writing, research, web site development - and so on. Apart from anything else, teaching takes up a lot of energy and you just don't have the same amount of energy at 50 as at 30. 25 hours in the classroom is easily the equivalent in energy consumed of fifty at a desk job.
Perhaps more.
So there comes a point when you've had enough. Exactly when that is depends on the person. I personally can't at the moment see myself "retiring". But cutting down on classroom teaching in favour of other types of work - I was ready to do that by 25.
If on the other hand you're thinking about a private pension and want to know when you should arrange for it to mature - I'd say as early as you can afford. If you're not ready to stop by then, you're not forced to, but if you can't wait - the money's waiting for you.