I think Hidden Love has handled first love so well. There's been so much chatter on this one for me personally it works because its well handled. The drama is aware of the age gap, and the ML never see the FL as anything but a sister, they have time apart she distnaces herself and says goodbye to her first love. When they meet again, the change in our ML is gradual and by then they are in much similar places in life - I think sometimes in age gaps timing is everything - 5 years in your teens isn't the same as 5 years in your 20's and 30's.
This drama is aware of that and therefore its working for me, I'm enjoying the gradual change, the growth in these characters. I'm sat and ready for more.