Currently children who are 18 or older may not be included in the qualifying parent's British ancestry visa to come and live and work in the U.K. This means that once a child turns 18 their right to explore their British Ancestry can be lost forever.
You may be interested in these active petitions
Children at 18 are often still dependant on their parents or may be in tertiary education and therefore are still dependant on their parents. The age limit of 18 on the Ancestry Visa can force children to be separated from their families.
The ancestry visa is a working visa that does not allow recourse to public funds, by extending the age there should be no financial burden posed to the U.K. and by extending the dependant's age to 21 it should encourage more people in of a working age to work and contribute to the economy.