Good question. Since COVID, they haven't been changing car age requirements every year, but that may be coming back. It seems to be based on the model year, not the rolling age (ie, 15 years old ages out on a specific date every year). U/L will announce that cars must be of a specific model year or newer. They just don't do it on a specific schedule any more. I know this because Lyft changed the standard early this year and it bit me. Here's what happened.
I drive a 2008. Sometime around January, Lyft decided that anything older than 2009 model year was too old to drive in most of CA. But, they forgot to notify people. They went ahead and disallowed those cars (they didn't deactivate drivers, just cars) without warning. They must have figured out their mistake because they rolled it back and reactivated 2008 cars after a week or two. (Well, at least they reactivated mine because I screamed bloody murder.) Then, around March, they sent out notices (presumably to anyone with cars in danger) that in about a month, they were changing the requirement to 2009 for all of CA, 2008 for some cities and 2007 for a couple rural areas, which they then did.
I seriously doubt that you'll be in danger of your car being deactivated around the holidays, especially on Jan 1, for a couple reasons. First, cars don't get a year older on Jan 1. They get a year older when the new models come out around now - Aug, Sept time frame. That's been the standard for a century. Second, making a change like that would adversely impact the driver pool for both U/L. They're smart enough not to do that during peak season. More likely, they'll do it in the Feb-April time frame to minimize the impact to their earnings.
Of course, there's no guarantee, they've both done stupid things before, but usually not to hurt their bottom line. Also, Lyft has already made their change this year, so you'll have them as backup in case Uber gets stupid.
Last piece of advice. If your car is getting that close to aging out, time to start looking at replacing it before you get caught by a change.