Yes it's normal to lose defensive skills with age. You made note that one player was marginal and that the other could only play 2B. That's a hint that they may not be useful for long. For non-starting position players your focus should be on good defense and multi-position ability. Most leagues should have a bucket full of marginal hitting but decent to good infield defenders to choose from. If you don't see that, check your player creation settings.
In my leagues I pay a premium for good backup infielders. They can save a season when injuries hit. Get them some playing time and occasionally the development engine will kick in a ratings boost.
Avoid infielders who were average to poor defensively when young. They will lose skills much earlier.