Quite a few official (government) and private jobs had age limits on initial entry - not too old.
These documents are probably associated with applications for official positions, as I doubt private employers would have required such documentation (thought not impossible).
As to why a parent had too provide this proof? That is easy, civil registration of births only started in 1864, so there were no birth certificates. Baptismal certificates were I think also usually acceptable, but these were likewise often unobtainable.
And how old were these two at the time of the declarations?
