I don't know if this happens as often in other countries, but this seems to way more common among Indian adoptions. I personally know of one family (that also dissolved the adoption) when a family adopted a "3-year old" girl from India who turned out to be at least 6 years older than it was stated (they found out when she began going through puberty at supposedly 6 years of age). I've heard of several other cases of Indian children being many years older than their adoption papers say.
From the Star Tribune.
A Minnesota couple were excited to become parents of sisters from India
— until they made a shocking discovery that raises questions about the
U.S. effort to stop international adoption fraud.
In court papers that paved her way to Minnesota, Komal is described
as a 12-year-old girl from northern India, eligible for adoption in the
United States.
She liked to assemble puzzles and briefly attended fifth grade, but
the 112-pound orphan displayed a violent streak that soon left a Mayer,
Minn., couple wondering if they were told the truth about the two
Indian siblings they spent $30,000 trying to adopt.
Within months of their arrival, and before the adoption became
final, Komal confessed: She was 21. Her younger sister, Shallu,
admitted she was 15, not 11 as advertised. The sisters said they were
told to lie about their ages and backgrounds by orphanage officials and
an India-based representative for Crossroads Adoption Services of
Edina, which handled the failed placements.
To read the rest of the story, click here.
Harlow's Monkey 
That’s scary, from so many standpoints. Lying about ages is very common. We have three children whose ages were all made up for adoption and are anywhere from one year to three years off. While some children’s ages are guessed because nobody knows, we came to find out that our transitional home director knew the birth mother personally and lied to make the children more “adoptable”. Sad for the children, confusing for the parents and just wrong.
Thank you for sharing this story with us.
It is very common in Ethiopian adoptions to have the children’s ages off by up to a few years.
I thought I knew what to expect about this story before reading it, but–21?!!!!