Started a few days after my 21st birthday. 1.5 years later I've got 14 fights. First one was after 5 months of training, and there was 6 months between my 8th and 9th fight due to opponents pulling out and an injury I had. So I've been very active with fights and I'm loving it. Do it! You may never make the olympics or be a world champion but you can still have some success and alot of fun.
I'm 25 and will have my first amateur boxing fight in a month. Granted I've been doing muay Thai and MMA since I was 18, but I've only had a few months of pure boxing.
Just look at bernard hopkins, that will answer your question. It's all about hard work and dedication. age don't matter unless your really old or really young