It's highly market-senstive what happens to them, but I have previously worked on dairy farms in NZ and AU, and in both countries, calves(except replacement heifers) that get raised at all are a rounding error, maybe a handful per farm that are kept for personal slaughter and consumption.

If the bobby-calf (aka veal) market is paying well enough, then they are shipped off for slaughter at ~4 days old. If the market doesnt pay well enough to justify 4 days of rearing costs, then they are killed immediately and sold for their skins.