UJM:
Once a couple of bochurim in the shiur are married young, it a) becomes NORMAL to get married young, and b) other bochurim will follow suit. The reason why bochurim start dating at 23 is because “everyone” dates at 23. Once a small, but significant, percentage start shidduchim, the rest will follow suit.
Always Ask Questions:
That Telzer Rosh Yeshiva wasn’t aware of recent data which shows that Chassidim have a lower divorce rate than other Yidden, despite – or perhaps even due to – getting married young. Had he been aware of that statistic, I doubt he would have maintained his position. Perhaps that Rosh Yeshiva would have agreed that in the internet age – where many bochurim become addicts between 18 and 23 – which itself causes divorces, we should get married at 17.