Malise Strathearn, Earl of Orkney, Strathearn, and Caithness - Same age as their parents.

(from the about, source is fmg):

JOAN de Menteith (-after 21 Mar 1367). [95]m firstly ([1323]%29 as his second wife, MALISE Earl of Strathearn, son of MALISE 6th Earl of Strathearn & his wife Agnes Comyn of Buchan ([1275/80]-before 1329).

[96]m secondly JOHN Campbell Earl of Atholl, son of Sir NEIL Campbell of Lochow & his wife --- ([1305/06] or [1313]-killed in battle Halidon Hill 19 Jul 1333).

[97]m thirdly (dispensation 11 Jul 1339) Sir MAURICE Moray of Drumsargard, son of Sir JOHN Moray of Drumsagard & his first wife --- (-killed in battle Neville's Cross 17 Oct 1346). He was created Earl of Strathearn 9 Feb 1344.

[98]m fourthly (post-nuptial dispensation 9 Nov 1347) as his second wife, WILLIAM Sutherland 5th Earl of Sutherland, son of KENNETH Sutherland 4th Earl of Sutherland & his wife [Mary or Marjorie of Atholl] (-before 19 Jun 1371).

Okay, now for Agnes Comyn:

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(Sorry, that's all I could find there, lol.)