How young can you be before you start experiencing symptoms of alzheimers?

I have a memory problem... I never had any real trauma to my head... beside the time when i was younger, my sister and I was playing and she pushed me into a radiator.. I had to get the back of my head staple closed... Now im 18 and i always forget random things... like i could literally be about to do something right now...and two seconds later forget what it is i was gonna do... or I could have my cellphone or bag in my hand... leave out the room for a second then forget where i put it at... What can be causing me to be so forgetful?