Sometimes I've been told I can be a slacker. 
That is not good. You should have better work ethic.
Do you have any suggestions for how I could do that?
Well the term was used for those that did not join the war. You could join the military.
Is there an age restriction? I might be too old. 
I am not sure but the term slacker goes all the way back to 1790.
I'm not that old, thankfully. Are there any other tips or tricks to help with self motivation or improvement?
You have to have good internal disposition. Also avoid negative incentives.