Kirby is 46 and already has a title and looks likely to win another. When Saban was 46 he had yet to win a bowl game or finish a season ranked.
Is Kirby ahead of Saban at 46? Yes.
Is UGA ahead of Bama as a program? Sadly, yes.
Is Kirby better than Saban? Not all time, but he's making his way there, slowly.
Th reason Kirby is so good at a younger age than Saban was (46 as you referenced above) is that Kirby worked with Coach Saban for so long and learned from him. Saban is still the GOAT but he is getting older. Kirby is at a great age. Unfortunately I think he will stay at UGA for a long time and win a lot of games/maybe a few more championships before he retires or goes to NFL.
I just want Coach Saban to win at least 1 more National Championship at Bama before he decides to leave the program in someone else's hands. I personally think he will coach until he physically can't. I don't see him retiring unless for medical reasons.