Term limits have been shown to create ‘brain drain’, and ultimately what winds up happening is that that legislators must focus on career growth - either spending their time in office campaigning for the next elected position, or looking to opportunities beyond politics. It takes time and experience to become skilled in crafting bills that don’t have adverse effects and cannot be overturned or lawyered to do things they aren’t intended to do.
The net result is that it creates a slew of amateur legislators, and professional lobbyists, as legislators are forced to retire just as they become skilled at the job.

An alternative to a retirement age is mental/physical fitness reviews, but that’s also tricky. If there isn’t a defined process then unscrupulous people will just use a doctor of choice to get the results they want, but if there is a process, politicizing that process to serve one party or the other could mean using mandatory retirement to force key vacancies.

I do think that at some point we need to pry the hands of people off the levers of power, and I can’t think of a way that is as ‘non-corruptible’ as a set age limit. It would not always be personally fair, but it would probably be for the greater good.