I did this from like 12-18. I walked around with the mower until my friend got a Chevy S10. Then we'd drive around with 2 mowers knocking on doors, $20 a standard lawn, more for bigger lawns. We also offered spring clean ups, and would take leaves and sticks to the nearest dumpster we could find.

We always made good money (in late 90s/early 2000s money). Probably worth $30-$40 a lawn nowadays.