The decline phase for players is pretty predictable. No players really last into their 40s and once the decline starts it's straight downhill and predictable. I would propose a more random decline phase. Each player would be given a rating or set of ratings which would indicate how likely they are to stay the same, decline, or super decline. You'd see these ratings with your advanced scouts and they'd only start to show at a later age (32?) so that amateur and intl scouting would not help you.