I know that but initially those with symptoms were told to stay home and self isolate. Testing was prioritised to those who needed them most, so there could have been plenty of 18-25 year olds who may have had it earlier in the pandemic but were never tested.
Given the difference between the official deaths and excess deaths, there are a lot of people who died without diagnosis; I'd imagine the same applies to those who contracted it without it being officially diagnosed. Given 18-25s were amongst the safest group, those with mild symptoms would have been less likely to be tested previously than other groups.