The crown age of a phylogeny is the time between the present and the moment of at which the first diversification (resulting in two lineages) happened.
Examples
check_empty_beautier_folder()
phylogeny <- ape::read.tree(text = "(a:15,b:15):1;")
get_crown_age(phylogeny = phylogeny)
#> [1] 15
check_empty_beautier_folder()