The UN defines a child as <18 and adult as >18.
On 2017-12-31 everyone born after 2000 would be <18 years old, but only for that day as anyone born on 2000-01-01 would be exactly 18 years old on 2018-01-01.
This does only go by the UN definition and not local countries domestic definitions of majority which can be anything from age 15 to 21 (and subject to change over the years)
According to available records since 2017-09-17 the oldest person alive globally is a Japanese woman born in 1900, so since then no one is left alive born previous to then.
It is true the statement above was only valid for a very short window of time in 2017 and there are also different views on exactly when a century starts (which have maybe mangled by everyone in most countries celebrating the new millenium on 2000-01-01)
https://www.seattletimes.com/nation-world/jamaicas-violet-brown-dies-at-117-japan-woman-now-oldest/
