A question that seems straightforward but gets messy fast. As a librarian, I've ordered international editions, and the ratings absolutely shift. The UK's BBFC gave the TV series a '15' certificate for its original broadcast, meaning no one under 15 could rent or buy it. In the US, it was TV-14 for broadcast, but the DVD box sets often carried a TV-PG or TV-14 rating from the MPAA, which is a different system entirely—more advisory.
Then you have streaming now, where platforms assign their own. Netflix might list it as '14+' while another service says '16+'. The early seasons are tamer, but by season five with 'The Body' and the darker arcs, the tone justifies a higher rating. Some European ratings boards were stricter on the supernatural violence, others more lenient. So no, not consistent at all; it depends entirely on which country's system you're looking at and whether it's broadcast TV, physical media, or streaming.
I find the BBFC's old insights online fascinating—they sometimes cited 'sexual threat' as a reason for the 15 rating, which the MPAA rarely highlighted. Makes you realize how cultural context changes everything.