It will make it more dense. It's still quite young so it's not a problem yet. The time to prune hornbeam hedges is August/September. You can do it with a hedge cutter, cutting the sides and along the top, keeping the bottom a bit wider than the top, not just straight up. I have one. Eventually it will get tall, even with cutting it and then it can be shortened by cutting the top off, but then mine is over 20 years old.