When you're considering adding to your family through adoption, the decision is even more complex. A 2-year-old who arrived in your family as a newborn may be less needy and better settled than a 2-year-old who came to you at 18 months. Attachment takes time, and certainly it is wise to help the youngest settle in well before bringing another child home. But there is no rule set in stone. You have to go by your gut and by what you know about your child's past.
Another issue that comes into play is parental age. Many parents are in their 30s or 40s when they decide to adopt. Certainly you want to give your children healthy, active parents, and that desire can lead folks to adopt several kids in quick succession. Parental age can also have a huge bearing on which country you can adopt from, and that in turn limits the age of the child you may bring home. For example, Korea's upper age limit is 43, and most of the kids coming home from Korea and between 6 months and 2 years on homecoming. Parents who adopt from Ethiopia can do so until the age of 50 or so, depending on whether or not they're willing to adopt an older child.
I've experienced a variety of age spacings with our 10 children. I've found that anything closer than two years apart is quite tiring to the parents. Once kids reach 2 or 2 1/2, they are often much more tolerant of babies' needs, and can even wait a bit to have their own met.
Our closest spacing, age-wise, is between our two boys who are 2-1/2 months apart, and who arrived home 18 months apart. The second boy arrived when the first was nearly 2, which was nice. That gave the first child a good amount of time to settle in. However, the extreme closeness in age meant they both often needed the same thing at the same time. My arms were always sore from the constant carrying of one or both boys.
In contrast, two of our Ethiopian daughters arrived home about 16 months apart. But because the first daughter was already 3-years-old when baby sister arrived at 6 months of age, the maturity difference was dramatic and the transition was much smoother.
Personality is another huge factor. Some kids are easy to please, predictable, and adjust to change gracefully. Others have intense, slow-to-change, or needy personalities that may benefit from more time as the youngest child.
So many factors come into play that it is impossible to make blanket statements about the "best" way to add children. You just have to take an honest look at yourself and at your children, and make the decision that feels the most right for your family. And no matter how carefully you make the decision, expect to feel some twinges of anxiety as you await the arrival of the next family member.