Wait until they pass 35 and find that getting up and moving about is not just an annoyance but vital to keeping your body away from a state of constant little nagging pains ;)
I can type with my thumbs but it's far slower than using my left thumb and right index finger. I am almost 50.
However, what does that have to do with 2 handed use? Thumbs typing is also 2 hands. Or you mean typing with 1 thumb? I see people, especially Chinese, writing on their phones with 1 thumb and holding it with the same hand (so 1 hand use); you have that too with swipe keyboards though for western languages. This us definitely not faster (or, for me, nicer) than just typing with 1 thumb and 1 finger.
Curious what other people do... I am on a tetras on a busy street now and wasting what (very) young people do: I see them all typing with 2 hands, two thumbs.
Not sure how that works with a lot of noise and people (subway, busy streets, airplanes etc); it seems to type out complete crap random people around me are shouting. So in reality touchtyping is faster.