If someone KNOWS that they will be getting into rebreathers then they are probably better going that way sooner rather than later.
Tech diving in OC does only a little to prepare a diver for RB.
I will quote for Richard Pyle, "A Learner's Guide To Closed-Circuit Rebreather Operations":
"vast amounts of open-circuit diving experience does not help one learn how to dive with a rebreather as much as a solid understanding of gas physics and diving physiology does"
A RB diver with 200 hours OC and 800 hours RB is likely more safe on a RB than one who has 800 hours OC and 200 hours RB.
Back to the original question, when to start tech?
While the transition can start at nearly any time with additional training ans study the diver still needs to build a substantial base of experience at each level before doing more advanced dives.
Nearly ALL skill needed for tech diving can be practiced and perfected in shallow water on air or nitrox.
There is not really any number of dives or years of diving that is enough but more a matter of variety of experiences underwater. Diving in different bodies of water, with different weather, different temperatures, different vis, current, no current, etc. Doing the same dive 100 times is not the same as doing 100 uniquely different dives.
Gee, I think I am rambling again,