I once implemented a variation of this where there was a column called 'data_type', the valid values were the various SQL data types, and in code I would do a switch() on the (string) value of that column and then cast the contents of the 'value' column based on that... Ah the folly of youth...

Depends

You could have

    property_id, user_id, value(string)
    1 (assume age), 1213,    60
    2 (gender),     1213,    female

or

    property_id, user_id, value_str, value_int
    1 (assume age), 1213,    null, 60
    2 (gender),     1213,    female, null

or have a mapping in the application to get the type of the property. Plenty of ways to handle it.