For the United States, is there a statistic of the age professors have when they are granted tenure?
If such a statistic is not available, is there a way to (more or less reliably) estimate the mean age from other statistics? For example, is there a statistic of the average age at which persons accept their tenure-track position in the US, plus the average duration of the tenure-track process?
Background of the question: When comparing systems of higher education, the age at which professors can approximately expect to gain lifelong employment comes up. For some countries this average age has been reported (42 for Germany, from data in 2003).
(Bonus Q: Does the statistic show a difference depending on scientific field or type of institution?)