Victor Hedman is the only Lightning defenseman who is under 25. Pavel Kubina, Mattias Ohlund and Brett Clark are 34, Marc-Andre Bergeron (an offensive defenseman and power play specialist) is 30, Eric Brewer is 32. How many more years do these guys have left to be capable NHL defenseman?
The Lightning signed Matt Gilroy, the Rangers' 7th defenseman, and Bruno Gervais this offseason. Gervais is very underrated and as a Rangers fan and resident of Gilroy's hometown (I played in his dad's basketball league), I can tell you that Gilroy is shaky on defense. He can be an offensive threat but relying on him for defense is a risk.
Victor Hedman has been shaky at times as well, but he is young and has an upside.
So then you ask if they have anybody coming up from the system to replace the older guys. And the answer, according to Hockey's Future, is no. They list the weakness of the Lightning's system to be a lack of young defenseman.
The Lightning have the 17th best system, according to the same website, and their top 5 prospects are all forwards. That doesn't bode well for the foreseeable future with players such as Lecavlier and Stamkos already there.
Well then who is going to replace the older defenseman when the time comes? Which leads to my 2nd point...