I would say for the first one "mainly players under 30" not "players mainly under 30" -- any one person is either under 30, or not, but not mainly under.
The majority of the players on our team are under 30. With 15 players on the team, at least 8 are under 30.
Our team is made up mainly of players under 30. With 15 players on the team, I'd expect that at leat 10 and maybe more a
If I am one person, I cannot be "mainly under 30." I am under 30. I am over 30. If I am 29, I am barely under 30. I suppose if I have replacement knees and hips, I am mainly over 30, with parts that are younger. Or if I have a heart transplant and the donor was 45, but I'm 28, I'm "mainly" under 30, with one small bit of me that is over 30.
The soccer team consists of players mainly under 30.
I thought about The soccer team consists mainly of players under 30. I don't like consists mainly of players under ....... as the soccer team consists of players regardless of their age.
We are not referring to individuals, but a collective group, the majority of which is unde