Requirements for International Adoption may vary from one country to another. You can check the Country Guidelines for more specific guidelines. Most countries allow the following:

single OR Married (at least two years is required by most countries)
in a second marriage (one divorce for each parent is acceptable)
on the 25th year of marriage (older parents are considered “experienced”)
parenting other children (either birth, step, adopted, or fostered) OR childless
between 25 and 55 years of age
older than 55 (for school-age children)
parents with different religions