The spell Time Ravage from Explorer's Guide to Wildemont explicitly states that its damage is caused by rapid aging.  Would this make characters who have a class feature that makes them immune to magical aging, like Timeless Body or Undying Sentinel, completely immune to the spell (obviously they'd be immune to its secondary effect)?

A normal target who succeeds on the saving throw still takes damage but is not aged, which means the spell can damage you even if it can’t age you. So characters who are immune to being aged artificially can still take the damage.

I get what you're saying, but the spells opening sentence is:

You target a creature you can see withing range, putting its physical form through the devastation of rabid aging.

This makes it appear that the damage the spell inflicts is also considered a form of magical aging and would therefore be prevented by such abilities.

I think it's kind of one of those suspending disbelief kinda situations. The spell does damage, the reason it does damage (the aging) is more for flavor. The spell doesn't specifically say it can't damage a target who can't age so it must still deal damage.

Notice... "If the save fails, the target also ages to the point where it has only 30 days left before it dies of old age. In this aged state, the target has disadvantage on attack rolls, ability checks, and saving throws, and its walking speed is halved. Only the wish spell or the greater restoration cast with a 9th-level spell slot can end these effects and restore the target to its previous age."

You still take half damage on a successful save, even without you aging, so if you are immune to artificially aging, you just take the full damage.

All right.  I was mostly just looking to see how much could be squeezed out of immunity to magic aging, since there are so few things that cause it in the first place it seems largely like a cosmetic ability.