Basically you can look at it from a financial perspective.
If the player is around 10,000 salary, even if he loses 30% of his value its still only a 3,000 difference.
So if you are looking at salary efficient players, you can afford to lose some skills during the season. Especially when you are talking about a salary of only 10,000. As you move up divisions, you may need to switch to younger players. However, in div 4/5 it does not seem so. The price you pay for vets is so cheap.
As for the drop it is mostly random. For instance I have 7 guys age 33-37 and I have experienced one drop each week in two weeks. 2/14 = 14%. As for if its worth it, it depends on the player and what they can mean to your team.
Basically whenever I hire a 33+ year old I am just looking at what he can do for me during the season. For the initial cost most of the time its worth the risk. It really depends on what his role will be. In my cases he has almost always been a new starter because I find it really hard to find good starters at decent prices. I normally do not have the patience to wait for the perfect player/situation.
I've found backups in the 29-31 age range normally. Because when I have the money to spend for a good starter I want someone who will be a better replacement than my current guy. Normally I can't find that unless I move the age up.