Please provide sources. Regarding 'until a certain age', from Simchas Torah 5715 it's clear that if it's introduced, it's from a certain age, not earlier than 9, if possible 12, and preferably not at all. Oholei Torah was born shortly thereafter. So a Mashpia would have to be asked not 'after that', rather 'before that'.
I understand there were exceptions and people got rare answers from the Rebbe to continue or pursue various secular studies. However, the Rebbe's Shitoh is clear that for most boys it should be avoided during their time in cheder/yeshiva, in which they should be totally immersed in Torah and Avodas haTefila.
I believe that the Rebbe would give different answers to different people.
Just as different schools were set up in different ways. There were a few bochurim who received a brocha
to go to medical school or law school whilst the majority dwelled in Limudie Kodesh throughout their 25+ years.
Some would continue, and/or go on Shlichus, and a few (rare) others would receive a brocha to attend schooling after marriage.
Otherwise, the definition of success is not in our careers,
it's who, how, and what we elevate when we are there. Whether in a Yeshiva, a Mesivta, in an ER, an OR, or in a courtroom..
Which brings me to the next topic -- the above "title"...
We are all shluchim in all aspects of our lives -- and that's even more apparent in today's day and age --
so, it's not an addiction (maybe an obsession? Daily way to live our life to bring Moshiach?) and it's not exclusive. It's everyone.
It's in every area of land, time, location, flight, vacation, etc...
(speaking of being "Jewish" on vacation - we had a funny St. Thomas response during an outing ---
The native island kids see someone in December with a beard - of any color - and they shout out, "Santa Claus!")