NetJets has spent a lot of money on lobbying pushing this age 65 rule. If they're really concerned about safety I believe it's within the contract for them to send older pilots to a company doctor for a through physical. But with the 3 year LOM plan, grounding a pilot cost the company around $250K-$300K in salary plus cost of benefits for 3 years. Including those I bet it's over $400K total cost. That's why I did the original post, I can see a fight over this if the rule passes. It's just not that hard to come up with something you can be grounded for, especially at that age.
But this is more about $$$, I think 75% of the over 65 pilots are senior Captains at or near the top of the pay scale. They'll be replaced by new hires. At the last union meeting the VP of Flt Ops attended he admitted they're having recruiting issues. A big part of that is indefinite wait times for Captain upgrades. Age 65 will help with that, at least a little.
As for if this rule will pass, it's an unknown right now. The big thing NetJets did to pay for this was endorse ATC privatization. That's been pulled from consideration, as has the reduction in the 1500 hour rule for ATP's. But they also spent a lot on lobbying so who knows.
The Union message board is filled with posts from pilots who are P.O.'d at the union for not opposing age 65. This is mostly just emotional ranting, Lawyers have told the union they must stay neutral or face DFR lawsuits from one side or the other. All the union has done is try to negotiate a 1 year "spool down" if the rule passes Congress. This actually helps both affected pilots and the company. They're already short of pilots, to have 250+ suddenly leave would be like dropping a nuke into the schedule. Lots of expensive sell-off's. The Union has been criticized for doing this, saying it's an endorsement but the Lawyer's don't agree.
I should have mentioned the optional Union sponsored LOM that kicks in after the 3 year company plan. It has been verified age 65 rule will not affect that policy.