That's a nice deer, but a tough question. There are so many factors that contribute to antler development, like heredity, amount and quality of available food, etc. I'd even say that what the weather is like in different parts of the country can effect antlers. For example, in states where bucks may barely survive bad winters, their bodies may be so deteriorated that much of the nutrients they eat during the springtime has to go toward rebuilding their bodies instead of growing new antlers. Where I live (southwest Montana), we have good genetics and, even though the winters can be brutal (for humans) there seems to be enough high quality food, and lots of it, so that relatively young bucks (2 1/2 years old) can grow some pretty nice antlers (4x4). Unless you've been watching that deer for several years, and have some idea of how old it might be, I'd guess
that in your part of the country, it could be as young as 3 1/2.
Here's a photo of a young buck I shot a couple of years ago. I aged him at 2 1/2. He didn't have a large body, but was in such excellent shape that he sported a fairly nice set of antlers. I can only imagine what he would have looked like in another year or two.