There aren’t many cars on the scheme for over 21s let alone under 21.
A quick search of mild hybrids on motaclarity for under 21s came up with the
Hyundai Bayon 42mpg, Hyundai i20 n line 54mpg and the Hyundai Kona 47mpg.
Electric cars: Fiat 500 Smart EQ 2 and 4 and the Kona @rox pointed out.