As far as B12 is concerned, it doesn't hurt to start to take it now.
Levels below 600 are considered deficient
(used to be considered 'normal' at 400-600)
Try to get sub lingual METHYL-cobalamin, not cyano-cobalamin, as the
methyl form is more readily absorbed.
I use sub-lingual lozenges from Jarrow (@iherb).
Relatively inexpensive at about 5-7 cents per tab.