the point is that Lukas is 20, barely out of high school, still very young and in a different age mentality/maturity than Joshua.
Lukas is trying to save up to go to college by working part-time.
Joshua has already done that; he's done at least 2-4 years of higher education and is working a career job.
Plus, everyone has different varying opinions and where they draw the line on age differences, especially when it's for romantic or sexual relationships. So while you may be fine and see no problem with it, others will not.
I hope that makes sense and got you to see things a little differently in another perspective. (●'◡'●)ノ
honestly that is not a big age diff at all like i seen worse 20 year olds with 40-50 years olds hes fine