I am strictly breastfeeding and have been pumping and storing milk since my baby was 3 days old. When he turns a month old, I want to introduce bottles with breastmilk so anyone else can feed him if I am not home. Is it okay to give him the milk from those first days of pumping? I understand those first couple of bags are mainly the colostrum. Does this matter once he's a month old, or is milk milk?
To some extent milk is milk, but the nutritional content shifts gradually over time to meet the growth patterns of a baby (and also again around weaning).
At about a month, your milk generally has more fat and less protein than breastmilk from Day 3.
Feeding him that milk isn't unsafe* or unhealthy, but perhaps consider mixing half Day 3 milk and half "now" milk. Not only will the nutrients be more balanced, but also the flavors -- if your baby gets a different taste in addition to the different nipple from a bottle, I think it will be harder to get him to accept it. (Two of my breastfed/bottle-fed babies had no issues with accepting bottle feeding, one fought it like crazy -- simplifying the transition can only help!)
* NOTE: This assumes the milk has been frozen properly. La Leche League has a useful table of acceptable storage times depending on storage method (countertop, fridge, freezer, deep freeze).
In case you really want to look at the nutritional components of breastmilk in great detail to help you make your decision, the science-heavy quotes below are pulled from an article by Ann Prentice, published by the United Nations University Press, Constituents of human milk.
The composition of breastmilk is not uniform, and the concentrations of many of its constituents change during the lactation period and differ between individual mothers.
Human lactation can be divided into four phases that differ in the composition and volume of milk produced: colostral, transitional, mature, and involutional. Colostrum is secreted for the first three to five days after delivery, transitional milk until the end of the second week, mature milk during full lactation, and involutional milk at the end of lactation. These definitions are arbitrary; the timing varies from one mother to another, and composition does not change abruptly. Typical concentrations of selected milk constituents are shown in table 3. Notably, colostrum is rich in secretory IgA, lactoferrin, vitamin A, and sodium compared with mature milk but has relatively low concentrations of fat, lactose, and vitamin B1.... Because milk volume is low during the colostral phase, rising slowly during the first week to the higher levels of established lactation, the daily intake of most milk components by breastfed babies increases after birth, reaching a peak after several weeks (table 4). The exception is ... total protein intake, which is maximal in the first week (table 4).
Mature breastmilk composition also changes during the course of lactation, although not as markedly as in the early weeks....