User blog comment:Kuopiofi/Saitama - Absolute or Supernatural?/@comment-24053170-20160907020749/@comment-24053170-20160907042413

Unless they are stated by the author directly to be so, then no they are not immortal and they do in fact age.

Also most of the clearly immortal characters don't actually require sleep or food, they merely do it so they seem normal to others and don't stand out as much. And this is actually stated in the backstory for most of those series.