This is a very controversial and contentious subject. To further obscure the issue, technology is changing.
I have personally spoken to the world's foremost authority on the subject, Skip Walker.
Here is a great article he wrote;
Photoelectric vs Ionization Smoke Alarms - Deadly Differences
Basically it says the following;
1) Yes, any smoke detector over 10 years old should be tossed and replaced.
2) Most homes, including brand new homes in The Villages, have Ionization Detectors. They pass code and work best for live flame fires. They are less expensive. Again, they PASS CODE. No, I cannot write them up as an issue. They pass code.
3) HOWEVER, read the article. The newest research indicates that more lives are saved each day with the slightly more expensive Photoelectric smoke alarms. Some places are now requiring them instead of the Ionization detectors.
Basically, if you are going to replace them, you may want to take a good look at the Photoelectric ones.....
Hope this helps....
Frank