There are services that do event collection and aggregation and then allow you to do a cohort analysis on a web or mobile app. (Like mixpanel)
Or.. you could be working with an already existing dataset in a DB or CSV and are looking for a way to split into cohorts based on some condition.
Any of these scenarios ring true?
I am actually looking at the second scenario (existing dataset).
If by dataset you mean CSV, you can always do a basic cohort analysis via spreadsheet software like Excel or Google Spreadsheets.
For a basic example, have a look at Daisy Deng's post on http://www.quora.com/Is-there-a-way-to-do-cohort-analysis-vi... , which links to https://docs.google.com/spreadsheets/d/1lD98hOCcpjsmxUmhUq8K... for a step by step guide.
If your data is in an actual database you can export to CSV first. Of course, you could always do the cohort analysis using SQL as well.
A third option for more advanced use cases is processing your dataset with R, which is powerful but takes some learning https://www.r-project.org/about.html
I wish there could be an easy option in mixpanel where we could pull some User Profiles (already on Mixpanel) and perform a Cohort Analysis on them.