How old are they? Look at the production date on the sidewall. It's 4 numbers in an oval, like 2609. The first 2 numbers are the week of manufacture (26th week) the second 2 are the year (2009).
Typically tires start to dry out after 4 years - no matter if they have any tread left or not. A brand new tire that has been stored properly would be the exception. Tread depth on a used tire doesn't really tell you anything.
4 years is plenty for an OEM tire, as they usually aren't that great when new.
I've replaced OEM tires with 500 mi on then because they sucked, and car tires after 3 years because they lost traction. If you haven't worn them out (don't ride until you see wear bars though) in 4 years I recommend new ones for the next season.
Plenty of good choices. I've had Continental SportAttacks and Bridgestone BT-016s (both sport compound) that were sticky and predictable but wear pretty quickly because they are soft.