It is ok to apply from Japan but you need to research in detail what they want not just what it says on the website. It is among the top most difficult embassies to gain DTV approval. Focos on showing long term use of the DTV to avoid getting an METV.
Your age is irrelevant plenty of 50 -70s have got the DTV, some switching from retirement visa. New businesses with no income have been approved for the DTV. You should show how you will support yourself while getting the business off the ground..e.g additional savings. Not necessary to use an agent unless you have probs with submitting paperwork
Bring the docs you applied with, you'll be asked general questions on your job, reasons for the DTV etc. You will need to show your current bank balance on your phone. It is an easy process and as the threads show, overwhelming majority are approved shortly after.