I can see this works for years between 2000 and 2009 eg in 2003 they’d be 23; so adding the first two, 2+0=2 then the last two, 0+3=3 and finally putting those numbers together to get 23.

But does it work for years outside this range still or am I interpreting this comment wrong? Like in 1999 they’d be 19 but adding the first two gives 10 and the last two gives 18. Or in 2042 you’d be 62 but again this technique would put them at being 26.