Well, the age of your fleet has almost nothing to do with aircraft condition. If they're regularly maintained, they will stay about 91% to 99% over the weeks and months whether brand new or 20+ years old. I think older aircraft will deteriorate more quickly when a maintenance is skipped though, but that shouldn't affect you.
The aircraft condition stat is quite volatile, in that you can easily rise and fall 30 or 50 places in the rankings from week to week. It's not one I ever even look at, to be honest. You can't see anyone else's individual aircraft condition numbers, nor can you average your own as the stat will be based on last week's figures which you can't see and will have changed. So you can't really predict where you should be.
I really wouldn't worry about it. If it's not calculated quite correctly, it will at least be the same for everyone.