When considering the youngest shell collectors you would have to include the
school children of Tortola in the BVI.  But I don't know if you would call
them "collectors" in the strictest sense, because they live with the shells
day by day--even in school.
 
When I visited  one of the rural schools there (albeit this was 15 years
ago), each child had a small pile of shells on the table in front of him--or
her.  Why? They were learning addition and subtraction using...what else but
seashells.  I have some prized photographs of the kids with their shells.  I
doubt if they have replaced them with computers yet.
 
I brought home from Tortola quite an assortment of limpets, olives, conchs,
bleeeding tooths (teeth?), flamingo tongues, astartes, cowries and others.
A wonderful time, a beautiful spot.
 
sj