Puberty (13), Election age (18), and Drinking age (21)
Puberty for Patau syndrome, caused by trisomy 13
Election for Edwards syndome, caused by trisomy 18
Drinking for Down syndrome, caused by trisomy 21
AND this list is in reverse order of frequency of trisomies resulting in live births (Down most common, then Edward, then Patau).
I always find this super difficult, but it turns out First Aid has a mnemonic for this.