When you're seventeen you know everything. When you're twenty-seven, if you still know everything you're still seventeen.
Unfortunately, most of us don't begin to think until the ripe age of thirty - when things are a little more quiet. The problem is, at this age it is too late to begin doubting one's basic assumptions in life. Pride is a factor - it is too distressing to admit failure and have to start all over again.
As we age we nearly always undergo a hardening of the attitudes, or a hardening of the heart - not unlike hardening of the arteries. All our efforts become directed towards extinguishing the idea that something new may be possible. Up until now we have had some ideals, some faith in reason, but age beats all the hope out of us. Now we need security, and security requires certainty. If our categories are not concrete and beyond doubt we will surely be crucified by them. Therefore, once we reach thirty we know everything - even if it is the cowardly certainty that we can never know absolute truth! From this time on we are the slaves of abstraction and unable to ask the noble questions of youth.