Smoke Detectors use a small amount of radioactive material in the detecting bit, over a period, as with all radioactive substances, this decays and becomes less effective.
10 years seems to be the usually stated lifetime, in fact many of the new ones are sealed with a 10 year battery in place.
Annoyingly my smoke detector batteries reached the low point around 3am this morning, and it's not just a beep, no it goes BEEP "Batteries Low", BEEP "Batteries Low", so out with the ladder and I pull it off the ceiling to find it uses 3 x AA batteries, which is a pain because they come in packs of 4.
Give it a clean, when I took one of mine down to paint the ceiling it was full of dust and grime, I blew the ionisation chamber out with dry compressed air and brushed the dirt off the circuit board. I have put this particular one on the landing upstairs as it is very sensitive to grilled sausage whereas another make seems to not detect sausages but works well with lamb chops.