I honestly don't see why his age matters. We really don't know his age at all, we are just speculating based on Naturo's age at the time of the Chuunin exam. But, we saw from the flashback that enough time had passed from Obito/Kakashi/Rin taking the chuunin exam to the 3rd ninja war, to allow Kakashi to be promoted to jounin and for Obito to take the chuunin exam a second time.
We also don't know how much time passed from his "death" to the attack on Konoha.

If his height is the issue for some, when we see Kakashi and the others during the attack on Konoha, they appear to be the same height as Naruto is currently. Obito would be of similar height.

If power is the issue, he was only able to do what he did because of an overpowered mangekyou ability. And he still did not measure up to Minato in any way. He challenged Minato, but he was overconfident and got his ass royally kicked and was forced to retreat. Even with an time-space ninjutsu superior to Minato's, he lacked Minato's experience.

I'm sure if you are patient, we will get some flashbacks that show what happened and answer most of the questions. Kakashi will most likely ask Obito many of these questions himself.