2002 Monte Carlo SS (21st Birthday)
 

Given that she just turned 21 years old, what needs to be done to make sure it is well maintained & reliable?

Age is only a part of the equation. Mileage and use over that mileage/ storage conditions between is another.

The main concern with age alone is rubber - getting hard, dry, and cracking. So that's all of the hoses, suspension bushings, mounts, sometimes seals.

Most of the wear and tear maintenance stuff is mileage based.

I normally watch mileage, but bumpin' is right. These cars have a variety of rubber parts and those can be a source of failures. But here is a thread that could be worth reading:
https://montecarloforum.com/forum/fw...0-miles-46078/