Well, for us we have the #1 OL in the league and 4 out of the 5 guys are under 28yrs of age. One of the biggest keys for OL is "consistency" (as any person who played OL would know, from Pop Warner to Pros) with knowing calls and tendencies of the man next to you. On DL, as long as you fit the system I feel it's easier to place players next to each other on the DL.
Don't get me wrong, both positions are VITAL to our success (as my coach used to put it, "Control the lines, control the game"), but as long as the DL can take up blockers, our defense is predicated on the play of our LBers, which is why we invested so much money into Willis/Bowman/Brooks/soon-to-be A.Smith. Justin Smith is my favorite player and is an invaluable talent on any team (basically a space eater/run stopper/pass rusher all rolled into one), but replacing DL in our scheme appears easier than replacing OL.
Remember the seasons after we got rid of Deese/Brown/Newberry in same season? Our best OLineman was Willing (a backup OG on any other team) until we drafted Staley, and even he wasn't great until Larry Allen joined the team. We have our first real franchise QB since Young, I think it's good if we invest more into protecting him