Not answering the general question, merely a solution to your concrete example:
[sql]
select age*10 as xyz,t.* from my_table join (select sum(age) * 10 from my_table) t [/sql]
Notes:
1) The sum is computed just once and returned as a table of cardinality 1. The table is then joined with my_table. The whole statement requires two full table scans on my-table.
2) (A x 10) + (B x 10) = (A + B) x 10