Food for thought...

That is a question riddled with more questions. First off, you have to define what a "decent human being" is, which is not that easy. Also, I don't believe there is any age attached to it, unless we are talking about maturity, in which case it varies. When you've reached your thirties, generally you are already shaped and defined from your knowledge and experience, or lack there-of. But many people never find that elusive thing they call wisdom.

Brandon's right, a definition is required, but here goes my generally morose view of it. the answer is never. Humans are a nasty lot.

I feel that it's all a matter of perspective. Whenever the time comes for one to feel that they're ready to become a "decent human being", they will become so. But it will only happen then, and not a moment before, because the concept of happiness and morality are all relative to the individual and no amount of coercion from others will change this. One must make a choice to become who they feel they should be--to accept that this is the how they want to live and exist. I feel like this correlates to the idea of enlightenment as well, for when an individual becomes enlightened, isn't it the same as becoming a "decent human", if decent means morally just. When one sees that it is a matter of choice then they will see that they have always been so, it was just time they needed to accept that it was true and not an illusion fueled by their own insecurities.

Oh, I forgot to say that I also think wisdom is something a person can only realistically hope to attain once they've opened their mind to any and every possibility without the pretense of doubt. In other words: wisdom will never be found if a person has any doubt that it's possible, and only when opening their mind to the entirety of the universe of possibilities will it fall into the palm of their hand.