How long do smoke detectors last?

If I had a question about them I would replace them. You can’t put a price on your families safety. Battery ones go anywhere.

At that age they are not likely to be carbon-monoxide detectors also, which is likely today.
A lot of smoke detectors can last that long if they are clean between the ambient air intake and the detector.
Most carbon-monoxide detectors have a tiny trace of mildly radioactive material that will be sub-performing in probably less than five years.

Maybe you can just put battery-op detectors where you currently have the plug-ins?

It depends upon the technology used. If they work optically they can last “forever”.
The ionizing /radioactive types last about 5 years.
Test them. Light a piece of paper, blow it out and let the smoke hit the detector. They should go off.

Dectectors are so cheap now you can replace them with a combined CO and smoke unit for $15–25. Have at least one from this century in the house.