Are you looking to take out a loan? If you are paying cash, there is no age requirement I know of. You should probably listen to your mother and hold off until you are 18. It's not like you could hide the bike even if you purchased it.
A note of caution to help you understand your mother: In my Navy days, I was the safety officer for my boat. The Navy kept outstanding records on sailors and accidents and one that caught my eye was that 95% of sailors who registered as motorcycle riders on base laid their bikes down within six months. I have asked riders over the years about this and every one of them has gone down, some multiple times. My bother went over the hood of a car that didn't see him and ran a stop sign. A man who worked for me was killed in an accident. A guy at work had a deer jump in front of him and suffered significant injuries. You are seventeen and I'm sure you think you'll be different. Those Navy guys were typically 18 and 19.