Credit goes to Pashos..You can max out your stats by doing well in Career mode. You get EXP for doing good things, like sweeping a series, winning 10 games, winning postseason games, and signing good free agents. You also lose EXP for doing bad things, like getting swept in a series or losing 5 games in a row. The EXP that you gain goes towards ranking up. You can view how much EXP you have by going to My Career. When you rank up, you gain points to use towards maxing out one of your stat categories. Choose whichever category has the most bars filled already and continue filling it up. You can fill up 2 bars every time you rank up. You should easily be able to get this while going for the 30 year career achievement.
When you start maxing out one of your stat categories, choose a category that already has 4 bars filled up and then you will be able to max it out once you reach level 10 if you only add to that category.