Danger #1:
1) You might fall in love with a teenager who will only grace your home with her beautiful spirit for a few short years before becoming a young adult and starting to branch out on her own.
Danger #2:
2) You'll miss her terribly when she's gone. You'll miss the chick flicks and late night chats... and you might even miss the eye rolling (if she's gone long enough).
____________________
Our 2 girls (who were unrelated 13 year olds at placement into our family) just got home from their 1st trip back to Ethiopia since we adopted them almost 7 years ago. They wanted to go back to visit relatives and to volunteer at the orphanage they lived in. (THEIR idea)
They wanted to paint the nails of the little girls and play soccer with the little boys. In their words - they wanted to "brighten their days" since they personally know how "very slow time seems to pass" when you live in an orphanage.
With their own money they bought tons of donations, toys and gifts to give away.
It was important to them to GIVE BACK.
I need emphasize that this was ALL THEIR IDEA. I want people to know this because, just like the adoption of HIV+ kids, the adoption of teenagers often gets a very undeserved bad rap.
Is it sometimes hard? Yes. Is there any adoption that is NOT sometimes hard? No.
Of our 5 adoptions ... an infant, a toddler, a grade schooler and two teenagers... I can honestly say that the teenagers have DEFINITELY NOT been the hardest.

There is no rule of thumb.
It just all comes down to the individual life experiences of the individual child. I've known people who have adopted international infants who have grown up to rebel against God and their families and basically broken their parent's hearts - and I've known people who have adopted teens who have grown up to become loving Christian adults who want to serve God and love others with their lives.
If God calls you to adopt ... answer. And don't get too wound up in the details.
When it comes to following God's lead... age, just like biology, is ultimately irrelevant.