The playing of a role, determined by age. Such as playing the age of a teenage son, even though the performer is long past being a teenager.
The taking on of a role, based on age. Where one will be required to mimic the characteristics of someone a specific age. Not to be confused with having an 'age' preference, this is more about taking on a specific role, based on age.
Where one party acts out as a child, the other generally taking the role of a parent or teacher, or other authority figure. Sexual acts occur, depending on the agreed upon terms prior to the 'session'.
This can involve nothing more sexual than a spanking on a bare bottom, to more involved sexual acts, including anal sex, bondage, and so on.
This is more in line of a fetish, but is part of the BDSM culture or accepted practices.
Generally it would fall under the Bondage / Discipline portion of the BDSM Culture / Community.
Practice (Associated Acts): Non sexual regression to one's childhood, for the opportunity to relive one's childhood. It does not involve any sexual connotation.
Age Play is not considered to be about latent pedophile urges, but about a desire to relive one's childhood, and engage in sex, that perhaps was denied or unobtainable when they were actually that age. It DOES NOT involve minors, though it might be have some legal ramifications in some countries.
It is extremely popular in the heterosexual community, in non BDSM confines. It is also popular role playing among some who prefer TWINKS.