The Lifetime Ladder is called that because most people will find an optimal level of fitness on one of its rungs and maintain that level, more or less, for most of their subsequent years. The ideal level to attain depends on your age, your general state of health, and the characteristics of your own body. This isn't about training to become a professional athlete--it's a common sense program to maintain reasonable fitness in the interests of health and overall well-being. There's no reason to go off the deep end striving for levels above those you're happy with, and can sustain on a permanent basis.
Almost all adults in good general health can reach and maintain rung 20 of the Lifetime Ladder, assuming they're not overweight or otherwise physically constrained. Most people below the age of 50 will have no trouble reaching rung 25. Beyond that, just continue to advance until you settle on a level you're happy with and can maintain every day. If, after a couple of months, the exercises at that rung begin to seem easier, your body's telling you its conditioning is continuing to improve. Try moving up to the next level and if it doesn't pose a problem, settle there.