Is this a mature hairline?

To determine if you are receding in the hairline, you should look at your hair pulled back and your eyebrows lifted high so that the creases on the forehead show up.  If I were looking at you, it would show me the anatomical landmarks to measure where your hairline belongs and more about what the shape should be. The mature hairline should be (generally speaking of course) one finger breadth above the highest crease in the furrowed brow with a gentle V-Shape. Without more photos, I can’t answer your question as your corner hair could be falling over the shape of the lateral edges of the hairline.