I like the Okudas' "explanation" for Cochrane: extensive radiation poisoning made him look older! Plus some alcohol abusers just look older. And when the Companion "revived" him, it was either to an "idealized" state, or what he would have looked like if he didn't go through a nuclear holocaust! Bit of a stretch, but what isn't with Trek continuity?!
As far as Cromwell being too old for the part, I seem to recall talk that the studio/filmamkers wanted Tom Hanks for the role. And Hanks was/is a big Trek fan, but it just didn't happen (scheduling? I suppose I could look it up, but I've never heard anything concrete saying he was officially offered the role). Could've worked, but may have overshadowed the regular cast.