Who gets to decide if it's "reasonable" for someone to give up a job - irrespective of their age?
If someone gives up work without something important (e.g. caring for children/elderly parents) to occupy them AND they expect the state to support them, they'd get short shrift from me. BUT if they have assets and/or a secure income stream so that they are able to financially support themselves, good for them.
I mean, I'd have thought their life would be more fulfilling if they had a focus or three to occupy them, be that an artistic or sporting interest, volunteering, whatever. But at the end of the day, if they're not being a burden on anyone else, it's no-one else's business.
The whole "living off other people's labour" is a vast oversimplification. If you run a business and employ people, you are living off other people's labour - but you have had to put something in (money, time, ingenuity) to be in a position to do that. If you own shares in one or more companies (and if you have a pension, you do), you are gaining an income based on other people's labour.
If you have money in a deposit account, the bank pays you interest. They're able to do that because they lend your money to other people for their mortgages and personal/business loans and receive interest from them, which they earn through their work - so the interest you are getting is being paid for by other people's labour.
To object to that in principle is essentially saying you want a communist system, where no-one owns anything. Or are people saying that profiting from other people's labour is OK on a small scale, but not if you rely on it for your entire livelihood? Why the difference?