Kim Anne
Some people seem to get through before their 20th birthday but they actually turn 20 before they finish training. It has to do with Qatar's regulations for serving alcohol.
They might not want to keep your offer on hold from September to January because a lot of things could happen. Having said that, I know of one guy who was 19 when he attended his interview in January but didn't start training in May when he had turn 20.
Is it IPAMS doing the initial screeing? Best thing would be to speak to them.