They do loose their strength with age. I have a Solarmeter that I use to tell me when a new bulb is needed. If you don't have one of these, I would change them out every 6-8 months. 6 months is usually what is recommended on the 5.0.
Yes but even that is not really correct it is just a general suggestion they can stop putting out the correct amount of UVB before 6 months or can last 8 months
I use double tube fixtures, so every 6 months I can rotate one new UVB tube in but also use the "spent" one for general lighting...it probably provides some UV, for a while. If one fails there is still some light until I get a replacement.