I don't think Igraine was that young when she had Arthur. In her twenties, yes, but that wasn't young to be having a child back in those days. I also don't think that Morgause is 'considerably' older than Arthur. I'd say maybe six or eight years older, but not too old. I don't think Emilia Fox is playing her age in any case.

Besides whether Igraine was barren or Uther was shooting blanks I think you have to give them a few years of trying to realise that the longed-for-child wasn't happening for them, enough for Uther to be so desperate he'd go to Nimueh for help.