This is a theory I kind of want to test out...
I have recently been irked by the notion of developing cognitive processes. For instance, as an INTP it is recommended that I develop and learn to depend on Ne. I think, however, that each person is assigned their 8 functions and from the start to the finish of their lives, those functions work in the same strength and same order. Personal development seems to me to be more a question of beliefs--having a belief system that allows you to take full advantage of what you've been given.
Let's say I tried to develop my inferior function, Fe. I could do my best do learn all about social expectations, what side of the dinner plate to put my fork on, etc. etc. My thought, though, is that this will never feel "right" to me, and at best will be heavily subordinated by my dominant function, Ti, and as such will never be as fluid and skillfully used as someone with dominant Fe.
For example, I've found that when I have the right belief system, I don't need to improve upon my inferior Fe, I just tend to get along with people and show them the best of myself. When I have a positive attitude about things, I tend to not care what other people think about me, become a bit more eccentric, and generally have a good time with people. No Ne, Si, or Fe development required, and it can happen at any age, as long as the beliefs are right. A child can be well balanced, as can a teenager or adult.
Balance isn't a question of trying to develop functions, IMO. With the right attitude, functions will automatically be balanced.