Check this Wikipedia article for more details, but, IIRC, Microsoft hired Dave Cutler and his team on 1988/10/31, so the NT project could in some respects be considered 20 years old tomorrow.
It took Microsoft ~13 years to deliver a mainstream user-relevant client version of NT (Windows XP was officially released on 2001/10/25), no doubt a lot longer than Bill Gates probably expected, twenty years ago, but what started as Windows NT is now the world’s most widely-used client and server operating system family.
An excerpt from the Wikipedia entry:
The main architect of the system was Dave Cutler, one of the chief architects of VMS at Digital Equipment Corporation (later purchased by Compaq, now part of Hewlett-Packard). Microsoft hired him in 1988 to create a portable version of OS/2, but Cutler created a completely new system instead. Cutler had been developing a follow-on to VMS at DEC called Mica, and when DEC dropped the project he brought the expertise and some engineers with him to Microsoft. DEC also believed he brought Mica's code to Microsoft and sued. Microsoft eventually paid $150 million U.S. and agreed to support DEC's Alpha CPU chip in NT.
(So, maybe the NT project is actually a bit more than 20 years old…)
Circa 2008, as explained at PDC 2008 this week, Cutler is working on the Windows Azure team.