Im almost 50 and still go at it pretty hard.
The biggest key to doing this business is build a route close to home. Fliers are cheap, put them in the neighboorhoods that you want to service, once a month if necessary.
Do not do homes on hillsides it will wear you down. Small yards are likely more profitable if your part time.
You will have to find the right "nich" for yourself, if your unsure at this time what that will be it will come to you.
Do not work cheap PERIOD. 10 small weekly yards done for 6 months at 50$ each will eaisly achieve your goal of around 15k in your pocket.
Oh and also have fun, while its hard work, it is enjoyable.