1. Age 11
You need to get a championship during their formative years, but at the same time, you don't want it to be too young of an age. The Bulls won their first title when I was just six, and it didn't mean as much when they were winning those titles, because it became expected. It was still great, but I feel it could have been greater. Also, the Cubs had a good year when I was five, but I don't remember it. Meanwhile, they had another good year when I was 14, and I cried when they got eliminated from the playoffs by the Braves. Is 14 a little old to be crying about sports? Well, it was either cry about sports or have sex with girls, and I had zero interest in the latter. That is why I feel that age 11 is a good balance. I think everybody finds loving sports way too much totally acceptable at that age, and it is late enough that you can fully remember and enjoy the experience.
2. Age 45
This is an approximate age, but it gives a chance for your child to share a championship with his or her child. I mean, once you're an adult, you're probably not getting the same sort of joy out of a championship, but sharing it with a child would probably be pretty boss. The first one is a no-brainer, and I feel like this is a necessity if they are going to have children. Obviously, without children, it loses some luster, but that probably still means that the kid is drinking heavily and can still enjoy a championship even slightly past his or her prime.
3. Age 25
There is something to be said for 70 years old as you can share it with two other generations, but at that age, you really cannot care too much about sports. At that age, health can become a major issue, so I just don't see the risk outweighing the reward. Meanwhile, 25 is that sweet spot for sports fandom. You are young enough to still care way too much about sports and not have too many other responsibilities, but you are old enough where you could actually afford to go all out in your enjoyment. I mean, if you are ever going to make a random trip to see your team in the championship, this is the right time to do it. There isn't a significant other and kids holding you back, and it won't be that hard for you to convince a couple buddies to join along in the adventure.
You may get more nostalgic about the other titles, but this will be the one that you'll have the most vivid memories, and that is why it has to be the third age that you would want your child to experience that championship feeling.