I've only recently started watching Reign, and, in my opinion, I find it an enjoyable show, (despite being historically inaccurate). I admit I'd prefer it if it had fewer sex scenes (they seem to over-do it quite a bit and I don't recommend it for younger viewers).
The politics are intriguing, but everything in the end comes back down to a girl's virtue, who slept with who, the king's mistress, the battle between Francis and Sebastian (the kings two sons) and their fight for Mary, Queen of Scots. Loyalty is slack, honour is rare, and violence is graphic and often unnecessary.
I admit, though, the show is oddly addictive. Many compare it to Vampire Diaries. I find them about the same, all in all. I recommend it for at LEAST 15 and up. 16, if you want to play it safe.