Any idea why HRH Markle and h are often referred to as young? ?

No idea - are they?   One of them certainly acts like a spoilt (rich) child.   And like William and Catherine, and the York girls etc., they do represent the younger generation of royals.

Adult male baldness can run in families and it's a condition that tends to affect men rather than women.    I (f) however, have inherited thin hair which does look rather sparce.

Young compared to 99 and 94.

To people in their, say, 50s or older -- much of the population -- those in their 30s are "young".

And plenty of people start balding in their 20s.  Prince William certainly did, and Harry had a bald spot at the back of his head in his 20s.

It's usually a good idea to pay close attention -- or to research intelligently -- subjects on which you want to post, unless, of course, you enjoy looking careless and uninformed.

They are young compared to the Queen and Sir Prince Philip - and even compared to Prince Charles and Princess Camilla. Prince Edward and Prince Andrew were "young" until really quite recently. 

Male pattern baldness affects at least half the male population by the age of 50. 

Young is "in." People under forty are described as young... Media perpetuates the  baseless infatuation with youth.