I would replace if there was any hint of a problem. This isn't like a $20 clock radio (if it doesn't work as intended, you're out $20 and buy a new one). Smoke/CO detectors are there to get you out of the house to save your life.

I like these, they are connected wirelessly so if one goes off, they all do.
http://www.homedepot.com/Electrical-Fire-Safety/First-Alert/h_d1/N-5yc1vZbmgbZkv/R-203658771/h_d2/ProductDisplay?catalogId=10053&langId=-1&storeId=10051
http://www.homedepot.com/Electrical-Fire-Safety/First-Alert/h_d1/N-5yc1vZbmgbZkv/R-203658772/h_d2/ProductDisplay?catalogId=10053&langId=-1&storeId=10051

I just replaced a Kidde combo smoke alarm/ co2 that we inherited in a house we bought 2 years ago. Same thing happened - started beeping and replacing the batteries made no difference. Maybe they are designed to break down every 5 years. I replaced it with the same thing because the base part was already attached to the ceiling.

How old?

Replace every 5 yrs. regardless ...

If so, could you tell me if mine are broken or are they still good?

All of them started beeping within 24 hours. I replaced the batteries in all of them and the beeping stopped. One of them died though so I bought a new one to replace it. However, I noticed that the new one had a different message when I pressed the test button.

The new one said "Fire Fire" and then "Carbon Monoxide, Carbon Monoxide".

The old ones only said "Fire Fire".

The new one is the same model as the old ones.

Any idea?

Thanks!