IIRC...
There's a limit of age 70 for 135/91k, but it only applies to large operators which conduct a certain number of legs each year.
Much like the original age 60 limit, it was politically engineered by managers of one company, Netjets. Coincidentally, only Netjets is large enough to trigger the operations threshold. So basically a special carve-out to get rid of older, expensive Netjets pilots.
With that said, maybe there needs to be an age limit for commercial ops but it should be applied evenly across the board. Or maybe just different rest rules, I'm sure many 70 year-old pilots who got a good nights sleep in their own bed in their own time zone are at least as alert as I am for a dawn report six time zones east of where I was the day before.