It is on a sliding scale really, certainly here in the UK. If you apply for sponsorship you will need at least A levels and probably a degree because the competition is so stiff. If you pay for your own training and are a little older, you can make up for a lack in academic qualifications by way of experience. Perhaps you have worked in a career and built up a track record for several years and now have good teamwork, management, leadership and other 'life' skills which a young academic may not have gained yet. This counts for a lot in the eyes of the airline recruiters, as does flying experience, so the more hours you have and the larger the aircraft you have flown, coupled with your 'life' experience, the less you need to focus on any academic failings earlier in life.
Don't get me wrong though, you still need to be able to handle basic Maths and English as some airlines will test you on these, but I wouldn't worry too much as many degree students are failing these airline tests....................do make sure your basics are good no matter what level of qualification you hold.
Hope this helps
PP
ps. I didn't get any A levels or degree and I've done alright.