Left tooth radical — pictograph of a mouth with teeth visible inside. In classical Chinese you read age from teeth (horse traders still count molars to date a horse). 龄 means 'age, years' precisely because dental wear is the natural calendar of a living thing.
Right 令 supplies the sound — lìng drifting to líng with tone change. 令 pictures a kneeling person receiving an order. Pure phonetic here; the command meaning has no link to age. Same phonetic in 玲, 铃, 零, 怜, all reading líng — a tight series.
