Probit Analysis

Hi everybody! I am new in your forum and I have a question for you. I hope you will be able to help me solve my problem.

I am trying to estimate the median age of entry for 9 skeletal maturity stages in a cross-sectional study of about 400 subjects. I know that probit analysis (with cumulative frequency) is the most appropriate method to calculate the median age of attainment of the various stages (not the mean), but till now I was not able to perform this test through various software packages (SPSS, STATISTICA, MINITAB, STATXACT and others). I know that this analysis is for dose reposponse levels and similar studies. All the software packages require a large number of input variables in order to perform the test, which in my case I don't have. The only input I have is the chronological ages corresponding for the various maturity stages for each individual subject in my sample.

Till now I found some articles that they used SAS to calculate the median age of entry, which software I don't have!

I would appreciate any help.

Thank you.