The youngest children will be in the USA and UK - for the UK if you aren't willing to do concurrent planning (you foster a baby where the case is likely to end in adoption, which will be by you if reunion attempts fail) then babies would be 5 months old upwards. 5 and 6 month olds are rare but you do see it. More commonly you'd be looking at 10 months-24 months and this is actually pretty common. If you did do concurrency, the children can be young infants, aged 0-4 months sometimes. Other pros to UK adoption is the process being defined with you knowing exactly what to expect at each stage and so less uncertainty. It's also free with minimal costs involved
In international adoption, it all depends on the country. They all have their own restrictions on a) which parents they accept and b) which children are available for adoption
Adoptions from the US happen, and this is adopting a newborn baby where the expecting mother selects you based on a profile of your family which is shown to her. The babies are all races but often African American, as some AA mothers percieve Europe and Canada as having less racism and therefore a good palce for their child to be. The cons of the US is that it will probably be a very expensive option and the time you wait is unpredictable as you are picked by the birthmother herself so you wait until one comes along who likes you, AND is OK with her child living on a different continent to her. unlike other countries which have more defined timelines. You could wait a couple of months or several years.
In other countries than the US, don't expect a child younger than 1 year, but I believe a couple of countries have children aged 12 months and older. There is usually a legal period a child must be available to domestic adopters before they become avaialble for international adoption
In terms of health, obviously there is unpredictability in adopting a young child, you just do not know how they will develop. For the UK and USA, a background of drug exposure in the womb is pretty common, other countries it depends strongly on the culture, alcohol exposure is pretty prevalent in EE countries. Learning disabilities are also a common background factor. Really, whilst you can know about physical disabilities the child has, and there are plenty of children without or with only mild physical issues, things in the UK/US and some other countries will depend strongly on your attitude to things like drug exposure, learning difficulties and mental health problems in the childs background.
Other things to consider when choosing a country is how much information on the childs background do you ideally want, what about contact with the birth family post adoption, what's your budget limit for IA, what about race of the child etc
We have several international adoptive parents here, nearly all adopted from EE countries (Kazakhstan and Russia) I think