As per immigration rules for dependents of Tier 1 migrants (para 319A-319K), a child must be under 18 years of age when he applies for leave to enter (entry clearance - from outside UK) as a dependent under Tier 1 category. See
para 319F and 319H(c) of Immigration rules.
A dependent child, over 18 years of age, can apply for extension (leave to remain - from within UK) if he was granted entry clearance as a dependent under the same category (in this case, Tier 1) and that he must not be married or in a civil partnership, must not have formed an independent family unit, and must not be leading an independent life at the time of the extension application. See
para 319H(c) and 319H(d) of Immigration rules.
regards