Line powered smoke alarm beeping (why?)

most have a backup battery, if not replace the unit

If it's that old, just replace it. Some units are powered by both 120 volts, and a 9 volt battery. When the battery is weak, it gives a signal, like a beep every 45 seconds

Line powered smoke alarms almost always will have a backup battery. Likely you need to replace that battery. Since it may be 16 years old, you likely should replace it. They become less sensitive with age and less reliable. Of course there is a very good chance that spider webs or dead insects may be causing a failure as well.

I would (and have in my own home) replaced the old ones. If you don't want to go that way, I suggest starting by vacuuming them and then taking a good look for a battery.

One other consideration.....

Since there are two basic types of smoke alarms, one which sees smoke with an optical sensor and the other which relies on ionization using a small radioactive sensor with Americium, the radioactive type essentially "wears out" after a number of years due to the half-life of the isotope. This type of smoke alarm should be replaced becuase it will no longer detect a fire, even if the unit appears to be working properly and emits no warning beeps whatsoever. Most manufacturers rate them for something like 7 to 10 years as far as I can recall.

Smarty

The half life of Am241 is 432 years so that likely isn't the reason.

when the battery gets low it'll still beep. even hardwired units.