You do know people in their early 40's can still train right?
Yea, and? You've already reach your peak by that age, it only goes downhill from there.
If you disagree with above statement, lets assume, for the sake of argument, that this logic doesn't apply to FT world.
Let's assume Gildarts started being a mage by around 18-20 years (if it's earlier, it just emphasize by point more). He's around 38-45 by Tenrou. Arc. This means, him training for 20+ years resulted in him being at a level which can mid diff Bluenote and Jura tiers.
Now, suddenly in 1 year training, he jumped up several tiers? I find that hard to believe logically. I just accept it, because it's the reality in FT. Relevant characters, whether they train or not, and/or whether their growth makes sense or not, WILL get buffed enough to be relative to other characters. Just because Mashima wants to keep his character portrayals.
Gildarts is just one example of this. there are quite a few more like this in FT
Let me make it crystal clear. I believe Gildarts is still the strongest FT member, bar when Natsu gets DF. I have zero doubt in my mind about it.
All I'm saying it it doesn't make logical sense to me. It's the way it is, because Mashima desires so.