I would then divide this category further down into two subcategories, the skyrockets, and the late bloomers:

The skyrockets:

Of which Bjarke Ingels is probably the most impressive case. By 25 he had already set-up his practice and started working on his first big commission — the notorious VM houses in Copenhagen — by the age of 29.

How did he do it?

A very clear vision of what he wanted and an extremely proactive attitude. He studied the greats early-on and modeled himself and his projects after them. He then adopted Rem Koolhaas as his guru and committed himself to working for him. He did land an internship with OMA and worked there for two years before splitting off.

Where his proactivity comes into play, is that he was also participating in open competitions throughout his academic career, thus building credibility and confidence in his skills.

The late bloomers:

Here, you have the likes of Louis Kahn who opened his office at the age of 50, or Frank Gehry who was producing pretty average projects until his 40s. Both of them had an awakening moment that changed the way they approached their work and liberated them from self-imposed constraints.