In most states adults can adopt other adults. For example, in 1991 Olive Watson (43) adopted Patricia Spado (44) as workaround for gay marriage not existing yet. Someone should use this to mess with geneologists:

Quentin, Robert, and Sven live in Maine and decide to adopt:

• Quentin adopts Robert
• Robert adopts Sven
• Sven adopts Quentin

The geneology graph is supposed to be acyclic, so I suspect genelogy programs can't handle cycles. Once you allow adults to adopt arbitrary adults, however, cycles are possible.

Luckily Massachusetts is on top of this problem:

A person of full age may petition the probate court in the county where he resides for leave to adopt as his child another person younger than himself... -- MGL c. 210 1

Update 2013-04-18: David Mandleberg points out that requiring adoptees to be younger isn't enough to fix this because sometimes people's ages change from the perspective of the legal system. For example, if your birthday is unknown for some reason you use a guess. Then if you find out what it actually is, you use that instead. It's not clear to me whether a Massachusetts adoption would be invalidated if it the adoptive parent later learned that they were actually younger than their adoptee.