Since I'm forty, I'll just ignore those last three words in that title. I've now enough experience to know what works and what doesn't, and how to do things profitably and well. The only thing I don't know is if I have enough stamina to essentially work two "jobs" indefinitely until I no longer need the "day job". In my late twenties I worked full time supporting my family while studying full time at university. In the four years that I did that, I thought it was going to kill me. I probably shouldn't expect any different with my startup efforts.