my own experience suggests, short term, the drops appear random -- i had a hot shot 33 year old SF drop 3 levels in OD in one week -- but those were the only drops he had all season. i've had other players not drop until 34.
but over a longer period of time the skills follows a curve (i'm not much at math, and can only visually describe it) that generally slowly ascends or accelerates. this describes the pattern of my one long-term experiment, a player that went from 120-something points to 80-something over 8 seasons, with the pace changing from one to three drops a season in the low 30s to 7 or eight by the late 40s. having a trainer with career extension is a must.
when i evaluate players on the TL, fwiw, i factor in a few drops if the player is 34 or 35, and several if older, and that the first skills to drop will be tangential to their core skills (inside players lose OD and PA, outside positions lose SB and RB, that sort of thing).
one other thing: i find no support for this view on the boards whatsoever, but i also think the skill drops are balanced in some ways by the rise in experience, so that these drops are not as catastrophic as i had once feared.