It's an interesting philosophic question OP, and interesting how many people are saying absolutely not. Many people seem to be answering as if you'd asked 'is there a law that says you have to volunteer in retirement' (obviously the answer being no) so is the logical conclusion that many people think our only moral obligation is to follow the law and so long as no law is being broken you can suit yourself? But then I think that isn't what most people think at all, the majority would agree that it's morally wrong to abandon your children for instance, or to cheat on your partner but these things are not illegal. And I don't think many people would agree that volunteering is actively a bad thing to do or morally 'wrong', in fact most people would say it's a good thing to do, and that in general we should do good things if we can, so why is it so many people instinctively don't agree with the proposition we 'should' volunteer when we have the time and ability to do so?
Personally I think it's a very similar thing to the response you get to threads asking if you should give to charity. For me it's twofold, one is plain and simple defensiveness, people do agree that morally they should volunteer but they don't want to do that themselves because it's more pleasurable to stay at home and watch TV (or keep your money and spend it on yourselves) than to go out and work voluntarily (or give your money away to the poor). I think that's perfectly natural human instinct myself but for the very reason we instinctively feel it's 'wrong' and therefore feel shame and guilt about it, and therefore the usual reply is to elucidate all the reasons why volunteering is difficult or impossible, or not useful.
The second thing is the proximity principle and the theory of crowd inaction. To give an example, if you were out for a walk by your local pond, and saw a small child drowning (no-one else is nearby to help), and you could with very little danger or inconvenience to yourself throw them a lifebelt, near enough everyone would say you of course should try and save the child. In fact it would be morally wrong, repugnant even, to carry on your walk as though nothing was happening and ignore the child's desperate pleas for help. And yet approximately one child dies a minute just from malaria worldwide, a disease that is easily preventable, it's estimated you can save a child's life by donating about £2000 - now £2000 is obviously a lot of money and yet over a lifetime I would say the majority of people could spare that. At £10 p/month you could do it in a bit over 15 years. So in a lifetime you could save 2 or 3 children. Yet very many people don't give any money to charity and don't see it as a moral duty at all. Partially this is because unlike with the child in the pond, these dying children are far away, in unfamiliar places and in cultures unlike our own, we don't know them. So while we abide stringently by moral rules that affect people we know and care about (e.g. not abandoning our children) and fairly well by rules affecting people near us and similar to us (e.g. not shouting abuse at our neighbours, not graffiting or littering our local streets), we find it hard to apply rules to far-away people and things. And also, while with the child in the pond we are the only one that can help, with the child dying of malaria there are millions of people that could theoretically help, so why should we be the ones to do it? So hence why although volunteering in retirement if you have the time and ability is a good thing, most people don't feel it as an 'obligation'.
Personally I think society would be better off if, allowing for differences in ability and other responsibilities of course, there was a much greater expectation on everyone to give both time and money to good causes. I'm not talking about a criminal law or terrible financial consequences if you don't, more along the lines of shared social responsibility. A bit like how we now (mainly) all disapprove of racism and homophobia, and while we won't necessarily put people in prison or socially excommunicate someone for expressing mildly offensive views, we do definitely think less of that person for doing so, and wouldn't elect them to a position of power or hold them up as an example to children. I think people that don't do at least something towards helping the less fortunate, within their scope of abilities of course, could be looked at the same way in the future, maybe?