Vintage matters because entry valuations dominate private returns. Funds that deployed into 2006 and 2007 generally struggled, while those with capital to spend in 2009 and 2010 bought cheaply and produced strong results with no difference in manager skill required.
Peer comparison should always be within vintage. A 14% IRR in a strong vintage may be below median while the same number in a difficult vintage is top quartile.
The practical implication is vintage diversification: committing a similar amount every year rather than concentrating, which is the private markets equivalent of dollar-cost-averaging. See dry-powder and j-curve.
Related: j-curve, dry-powder, dollar-cost-averaging, private-equity, venture-capital, internal-rate-of-return