ONS says

"The number of younger people in England who have tested positive for the coronavirus (COVID-19) has increased in recent weeks.This has been among people aged 17 to 24 and 25 to 34 years, while positive tests for COVID-19 among those aged 50 years and over appear to be stable or declining".

So, whilst more younger people may have been tested, the stabilisation of older positive tests suggests that new infections are among the younger population.