I tried to answer this (re: why lass is not commonly used, but lad is in the UK) in the random thoughts thread in et cetera, as I couldn't post in the thread where this was brought up. Unfortunately, it got buried........in random thoughts, so might not have been seen by many of you. So thought I'd bring up in a separate thread. Lass is generally considered archaic in most English dialects in the UK. It's still commonly used in Northern England (especially the geordie and mackem dialects, or Newcastle and Sunderland respectively, and Cumbria) and throughout Scotland. Whereas "lad", particularly for younger men (hence why I picked my own user name back in t' day, and I'm from the West Midlands not Yorkshire, so the use of t' was more for poetic licence) is still commonplace throughout the UK.
I realise this isn't a question, just thought I'd answer it anyway.
I realise this isn't a question, just thought I'd answer it anyway.