Let's suppose you have your own company and you want to hire developers

Would hire a junior or a senior ? As a first step

If your answer is senior
Please tell me when it's the best time to hire a junior ?

It depends on so much, a junior might grow into a senior within a year but others stay junior for 5years. Some don't need constant help, others do. Just go with your gut feeling,

it all comes down to the time you are able to sell. Rule of thumb says that you alone should bill 60% of your time. 40% is administration, bookkeeping and aquiring clients.
If you have projects that requires more time than those 60% of your time you either hire someone or decline the project.

So If your Senior gives you 90% billable time and he drowns in work then it's time to hire someone new. As long as you can afford it. Remember the junior needs care too which means less billable hours.

You seem to have lots of questions regarding seniors.

Would I be correct to assume you are a junior but think you deserve to be a senior?

@Codex404 so when you would hire a junior or a senior as a first step and when a junior?

@neodite yes so true

@Mba3gar it depends

@Mba3gar how much work do you have on your plate? Are you making a profit?

@heyheni am not asking for myself i already know my worth.
But in my country all the available jobs are seniors