" guessing that you started using the Hanen book and Teach me to Listen and Obey by Laura Mize at around 18 months when the problem was identified and you would credit those techniques with leading to the language explosion at 2 and 3.5 rather than it being that they simply weren’t ready any earlier and would have always bloomed a bit late. That’s an impossible question to answer I know!"
With DS1 there was nothing formal, no intervention, I just thought about it a bit but didn't buy any books or anything. noone got involved.
With DS2 I started using the books/advice at around 3 years.
I would not credit either child's language explosion to any intervention whatsoever. I suppose conceivably the "clearing the ground of bad habits" might operate to unblock or unkink the pipe, as it were. But the language juices were always there, trying to flow through the pipe.
Whereas there are some children with whom you could intervene perfectly from day 1 but there will never be a language explosion.
Biological potential plays a big part.
What I absolutely credit to intervention is the fact that DS2 is pretty well-balanced, well-liked and not anxious. In contrast, my brother (similar biological starting point) is lonely and tends to monologue about his special interests and alienate people :(