Why do people keep saying the MC is 50? He was 37 when he died. Mind, "middle-aged" starts a lot younger in the youth-obsessed Japan, and the obese version of the MC is certainly drawn to look a lot older than he was... but still, he's not 50, unless you count up his age post-mortem and continuing into his reincarnated life, so assuming he's 7 (or twelve? I'm finding these time skips confusing) his spirit at least should be 44 (or 49, if his current body is 12).
In any case physical age doesn't make for maturity, experience does, and this guy was a shut-in for who knows how long. The thing I find the hardest to believe is how well-adjusted he's acting, hikikomori tends to originate from severe trauma, and whatever *that* was was never brought up nor resolved in the plot so far.