Seems to me that Singer and worse, Kinberg, just don't give (too much) of a darn about making things make sense.
There is no way that Angel should be a teenager to young twenty-something in 1983 unless he was brought back in time, which is very unlikely. But Kinberg wanted him in the movie so he just shoved him in with no concern for the timeline. And in my opinion that's terrible. Yes, we can look the other way like they are doing, but the point is we shouldn't have to!
They actually fixed a bunch of Last Stand crap with DoFP and now they are making more problems, and that's too bad. Hey, it is what it is, but IMO if they are willing to do something like this, which further wrecks the movieverse timeline, then at least make that character (in this case - Angel) vital to the story. And the moment I saw that Magneto is a Horseman it became evident that Angel/Archangel is going to be seconday and will likely not receive the focus that the Horseman Death should receive. And again, that's too bad!
What a missed opportunity!