How can I make this query work?

SELECT (AGE * 10) AS XYZ, SUM(XYZ) FROM MY_TABLE

There are several ways you could try this. The easisest however (but probably not the most efficient) is to use a subquery.

SELECT
(AGE * 10) AS XYZ,
(SELECT SUM(AGE * 10) FROM MY_TABLE ) AS XYZ_SUM
FROM MY_TABLE

This is specific to MySQL. This will give you a running sum. The last row will have the total sum of all the ages.

SET @a := 0;
SELECT
(AGE * 10) AS XYZ,
@a := @a + (AGE*10)
 FROM MY_TABLE

Not answering the general question, merely a solution to your concrete example:
SELECT age*10 AS xyz,t.* FROM my_table JOIN (SELECT sum(age) * 10 FROM my_table) t
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