Increasing evidence suggests that He ii proximity profiles in the quasar spectra at z \sim 3 - 4 are sensitive probes of quasar ages .
But the development of their H i counterparts is difficult to trace and remains poorly constrained .
We compare the UV spectra of 15 He ii quasars with their high-resolution optical counterparts and find a significant correlation between the sizes of He ii and H i proximity zones .
The luminous quasar HE2347 - 4342 displays a null proximity zone in both He ii and H i , suggesting that it is extremely young ( age < 0.2 Myr ) .
Three other quasars also display small proximity zones for He ii and H i .
There is no evidence that a H i ionization zone expands considerably faster than its He ii counterpart .
The results suggest that the expansion of quasar ionizing fronts may be noticeably slower than the speed of light , and raise the possibility of distinguishing young and old quasars from the sizes of their H i proximity zones .