Since there is no reference to him in Three Against the Witch World, he must be younger than the Tregarth triplets. I suspect he wasn't born until after Kaththea was kidnapped by the Witches (17 yrs old). When Koris recovered from being "sorely wounded" and returned Volt's Axe to it's tomb (it probably no longer worked for him because of the broken vow to protect Kaththea), I suspect Loyse decided it was finally time to have kids and Simond was born. The triplets were probably 18 when he was born.
Simond has to be at least 16 (he's described as "young") in "Sand Sister" when he meets Tursla (who was of age to bear children by this point) and they fall in love.
This probably takes place shortly before their appearance in "The Warding of the Witch World" because they've only been married a short while there.