I think you can start now. If you've built enough to have a decent portfolio to show, then you should be able to get clients based on that alone. Team up with a few people that can make up for any weaknesses you might have and get to it. IMO design skills are huge since the way something looks is often how clients will judge your work, so find someone who can wireframe and mockup solutions quickly and get your initial clients excited about your work. You can start with smaller projects and build your way up.