The age of the solar system has been known for about four decades and is now known with reliability and precision more than adequate for most purposes. Despite great effort, however, there has not been similar progress in the conjugate question of how long it took to make the solar system.
There is a general agreement that the sun was made by the collapse of a cool cloud of interstellar gas and dust. The "solar nebula" was an extended disk structure through which most of the solar mass was funnelled; its debris constitutes the present planetary system: planets, moons, asteroids, etc. Processes in the nebula are thus responsible for the character of the present system. The duration of nebula processing is clearly a major constraint for any model of nebular history, but there is little consensus on this central issue; proposed timescales range from less than 10^6 to more than 10^7 years.
Theoretical treatments of nebular dynamics usually favor the shorter timescales. There are a handful of isotopic-geochemical techniques, applied to the least modified nebular remnants available (i.e. undifferentiated meteorites), which indicate the longer timescales. The isotopic timescales can be dismissed only by making assumptions which are fraught with complex consequences. A third approach involves astronomical observations of very young stars; such observations are not unambiguous but seem to favor the longer times.
There has been a tendency for isotopic geochemists to adopt the short theoretical timescales as a constraint in interpreting their own data. This is unnecessary. Nebular evolution is a complicated process for which there is still no comprehensive first-principles theory, whence this seems a case where theory should be guided by observation.
