If you're not breeding then frankly I'd get bullocks. (Mares / geldings
. Plus female bovines may jump a lot of fences to try to find a bull, and will come a-bulling every 21 days forever... Though usually quieter over winter.)
If you are planning on keeping literally as pets, ie., not slaughtering and butchering and replacing with youngsters every couple of years, then you better choose a small breed!
Buying a pure beef or rare breed you will probably only be able to buy weanlings, so choose a seller who has done a lot of handling with them. Beef calves normally get weaned quite late, around 8 months, but you may be able to arrange to get some castrated boys weaned a bit younger. If you keep them in over winter, they'll get pretty tame anyway
.
If you buy beef cross bull calves from a dairy farm, you can get them at a young age and bring them up yourself, so will get them very tame. (Resist all games which involve them pressing their head into you in any way shape or form
. When they weigh nearly a tonne, you don't want them butting you, even in affection!)
A lot of dairy farms will put some or all of their cows to an Angus or a Hereford, calves of either will be biddable and make nice pets.
If buying unweaned, don't get ones under one month; if they make it to one month they'll probably do fine.
I usually keep them on milk to 4 months old, though many wean them sooner. They should already be eating straw when you get them, and you can introduce them to grass straight away - but gently so their systems get used to it.
I get them on eating cake from about 6 weeks, just a little, then by the time I want to wean them they really like cake. Giving them cake but no milk is not such a big deal
. I usually stop one milk feed first, then the second, so they are already used to eating cake when they are hungry for milk.
I keep them on cake for a couple of months after weaning if I can, depending on the time of year and the grass. Your grass will be a heck of a lot better than ours, so you probably wouldn't need to cake as much or for as long - but with ruminants, make any dietary change gradually, of course.
If you buy weanlings they'll already be castrated; if you buy babies direct from the dairy farm, you can ask them to get them castrated before you take them, or get the vet to burdizzo them for you while they're still small enough to restrain with a couple of people and a gate.