In thirteen countries, 21 is the age of majority. See also: Coming of age.
The number of spots on a standard cubical (six-sided) die (1+2+3+4+5+6)
There are 21 trump cards of the tarot deck if one does not consider The Fool to be a proper trump card.
The number of firings in a 21-gun salute honoring royalty or leaders of countries
It is very often the day of the solstices in both June and December, though the precise date varies by year.
This was a list of odd (potential) significances I found regarding Ouma's birthday before I ended up streamlining to two of them here, so I thought I'd pull them up and share those too.
The significances? Let's see...
'Age'- at times is wiser than he appears, if mostly acting childish
'Six-sided die'- a nod to his group DICE, idea explained in link
'Tarot deck'- predictions, maybe even supernatural gift a la Junko
'Royalty/leaders'- he's a leader, his name (and lab door) has 'king'
'Solstice'- the opposite solstice to Junko/Mukuro. Consider, an opposite to Junko, despite the story labelling him a 'follower'
Well, that's just from a glance anyway~
