Good answer. The compute method has to work on both the current record as well as a group of records, since in Odoo there are ways to edit muiltiple records (Multi-edit, Import, Server Action, etc).

Thanks Cédric and Ray ,but is that editing all records is efficient ? Imagine i have 10 Millions records ..... so why the compute function is going to compute 10 Millions times (whether the field is stored or not)?

Ideally, you wouldn't be iterating on all records on a certain model all at once all the time. You would iterate over the subset (recordset) of records that had a change in the "age" field. In any case, computers are very fast at multiplication (and basic arithmetic operations in general) and, while I've never dug through the database interface, I'm almost certain Odoo's ORM has a batched read and write mechanism to retrieve and change the whole recordset at once in the database, reducing the overhead cost of I/O for the operation. If you think about it, every time you open a view, you have to fetch all the records with all their stored fields to check against the domain and filters to keep the desired one. That's a lot more read operation than what you're doing here.