It is difficult to age that deer because you can not see the whole body, which holds some features hinting at the age of the deer. I would say, looking what I can see, my professional guess would be between 2.5 and 3.5 years of age. Again, it is very difficult to say since I cannot see the rest of the deer. What does the tooth wear look like? There are some webpages on the internet, posters, and jaw molds that can help you determine the age of that deer and future deer. I would recommend that you purchase one of those and read up on the info for ageing deer both on the hoof and by tooth wear (both are not 100% accurate, but are the most economical($)).