So they should just age out in poverty and die? Such people exist. To be clear, I'm 1000% against anything resembling a draft, but if an older person wants to, why stop them? A guy in my brother's medical doctor graduation class was 46 years old. Good thing nobody explained to him it was too late and he failed already.
In all seriousness, I do agree about the functioning knees part. But as long as it's voluntary, I don't see the downside.