There is no hours requirement in JARs to start the ATPL theory course, though each student must hold a valid PPL issued by an ICAO nation. Some of my students have fewer than 150 hours, and I teach ATPL groundschool. The 150-hour requirement is to start a CPL course module, or perhaps a requirement CCAT themselves make, though I cannot think why they might.
There is an age restriction to hold a CPL (18 years) or an ATPL (21 years) but none to sit a course per se (except of course the minimum age to hold your PPL, as that is a course requirement. It gets complicated!).
I would like to point out that apart from low hours each advantage of the integrated course suggested by FFF also applies to any modular course if all modules are taken with the same flight training organisation (FTO). As FFF says the "advantage" of a shorter flying course is a disadvantage when applying for the type of job most of us will start in.