Well, at 19 yoa, he was playing SH for England U20 (and Bath academy); at the same time as Robson was 18 yoa, playing SH for England U20.
Quite honestly though, very few players appear on my radar before they reach U20 level. I don't recall any talk of Cook converting to SH on joining Bath, though it is possible.
Either way; I'm saying I wouldn't advise it; not that it's impossible and has never happened.
ETA:
On Cook - his England U18 profile lists him as a
scrum half; as did his
SW U18 profile aged 17
On Robson - I've found an interview with him aged 18 being "equally comfortable with playing him at
scrum-half and fly-half"