(This is a bug for MCPE. When I clicked on the dropdown bar on 'project', it only shows me 'Minecraft', 'Scrolls', and 'Minecraft API'. I hope you can forward this to the MCPE side! Thanks )
Anyway, 'old' worlds were supposed to have the 'old' terrain generator. It should not generate flowers and 1.7 trees. 'Old' worlds were supposed for older devices but it seems like 'old' worlds had the 'new' generator!