Ben torah:
There are three factors which must be discussed for every solution.
A. Will it actually make an appreciable difference.
B. Is it implementable; will people do it?
C. Will it create other problems; will it be worth it?
So, in regards to your first idea, I’ll say it satisfies the first question but fails the second and third, with no further explanation needed.
Your second idea, everyone getting married at 18, bears discussion. I will divide my response into three categories corresponding to the three issues.
A. If effectively implemented, it would eliminate the age gap.
B. Implementing this could be very difficult, especially in light of the potential dangers.
Additionally, there really is nobody who would be able to convince the masses to adopt this custom. We must remember that the cohort we are dealing with does not have rigid definitions, but is a fluid group.
Also, creating this custom would likely alter the composition of the group, as many who consider themselves part of “Yeshiva Orthodoxy”, would not be ready to adopt this lifestyle.
So, the feasibility of this idea is very questionable.
C. This is the most important, as it is the basis for why nobody would want to do this. This solution would create several problems.
Firstly, most 18 year olds in our community are not mature enough for marriage, meaning the relationship. Pointing to the success among chassidim is disingenuous, as their lifestyle is quite different. We could change our lifestyle, but that brings us sharply back to “issue B” (namely, we’re not going to).
Secondly, currently, men in the litvish community learn many more years on average than chassidim. The standard chssidish man leaves learning about 2 years after marriage, at around the age of 21. (I may be slightly off, I’m sure I’ll be corrected, but I’m basically on target.)
It is very likely that our adopting this custom would lead to a similar situation, as parents have only limited resources towards supporting children in learning, and at 18, women will necessarily not have the same advanced degrees. (speech, special ed, graphic design, nursing, shaitel maching)
Also, it is verifiable that the amount of years one learns is by and large, dependent on when one gets married, ie. 3 years after marriage + 5 before = 8; 3 after + 2 before = 5.
Thirdly, we will be alienating a large segment of our greater community. The same boy who now learns in our yeshivos for several years yet comes from a more “baal habatish” background, and eventually assimilates into more “yeshivish” culture, is not ready to commit to this at 18. (BTW, that means less boys)
The same is true for girls. Girls from less yeshivish backgrounds whose communities will not have joined the new group, will not wish to marry 18 year olds from the new group.
What will result is a new, thick line, in “Yeshiva orthodoxy”, dividing it into two defined communities, instead of the continuum which now exists. This would be a bad thing.
I will probably think of more problems, but I think these suffice, and this may be the longest post I have ever written. Nobody is going to read it.