Hi JessicaPaige - As others have said, probably the most likely route to adopting a very young child is through early permanence. There are two routes to early permanence in England - concurrency and foster for adoption. With concurrency, you would be dual-approved as foster carers and adopters from the outset, and children will be placed for whom the outcome is not yet known. They tend to use concurrency for children where there is a some possibility that they might return to birth family, so relinquished infants etc., although not if they think there's a very strong possibility of return. In some cases concurrent carers are able to bring a child home from the hospital soon after birth (I used to be a foster carer, and was once down to foster a newborn who instead actually went to a concurrent placement). So, with concurrency, it's often the case that assessments etc. on birth family members are not yet complete, and the outcome of these may mean the child returns to birth family. If you think you can take the uncertainty and you have no other children, it's worth considering. Concurrency moves the 'risk' from the child to the adults involved, minimising the number of moves a child might have to make, while obviously increasing the uncertainty for the adults.
With foster for adoption, the adopters are temporarily approved as foster carers for a particular child with whom they have been matched, so that the child can move in with them before care proceedings are completed. This is usually used in cases where a return to birth family members is extremely unlikely, so perhaps assessments have already been carried out etc. and therefore the chance of the child returning to birth family members is lower. I would say, anecdotally, that F4A placements are less likely to happen as first placements - often the child will be in a foster placement while initial assessments are made, and then moved to pre-adoptive F4A placement when it looks likely that adoption will be the outcome, but moved earlier than they would otherwise. I see this used a lot where a family is adopting a younger sibling of a previously adopted child. In both routes, it is possible that the child would still be having some contact with birth family members so that's something you would have to bear in mind - with infants where the outcome is not yet known, this contact can be as often as 3-5 times per week.
Part of the reason that few very young infants are adopted is because of the timescales involved in the court and social work processes. Once a child is removed from birth parents, LAs have 26 weeks to finalise their care plan. When I was fostering very young children, I never experienced it taking less time than that, and sometimes it overran and took quite a bit longer. Where birth parents are willing to engage with the process, there will often be a period of assessments, opportunities to attend courses, counselling etc., so the 26 weeks gives some time for that to take place, and also for wider family members to be assessed so that children are not being adopted when a relative could have cared for them. After the 26 weeks, there is of course family finding and matching to be done. The youngest foster child I moved on to adoption was 9 months old and she had come to me at 3 days old - that was a straightforward adoptive placement after care proceedings were completed uncontested.
In terms of whether it's more desirable to adopt a very young child or a slightly older child, well that's very much down to you as an individual. As others have said, with infants, there is no guarantee that there won't be difficulties ahead, and you will have very little information about what those difficulties might be. FASD, for instance, can't be diagnosed until a child is much older, unless there are very obvious facial features present. Lots to think about!