Open their card, press Edit and pop the date into Born. Only know the year? That's fine – every date is "(if known)", and nothing on the tree is ever compulsory. Once a birth date is in, their age shows beside it ("age 38"), with no maths needed. For example, if all you know is that Great-Grandad Albert was born in 1902, just put 1902 and fill in the rest if you find it.
Where to find it
Person page → Edit, far right
Only people with the "Edit" tick.
On a phone: if the card's buttons are tucked away, press ☰ Menu first.
Still stuck? Talk to a real person: send us a message, or email support@mywholefamilytree.com