Case statement to determine if I should union

You could use an ugly hack something like this, but I think Tim's answer is better:

SELECT  age, nameFROM    usersUNION ALLSELECT  25, 'Betty'WHERE (SELECT COUNT(*) FROM users) > 1;

If it's in a stored-procedure you could use If...Else:

IF (SELECT COUNT(*) FROM users) < 2 BEGIN  SELECT  age, name  FROM    users  ENDELSE  SELECT  age, name  FROM    users  UNION ALL  SELECT  25 AS age, 'Betty' AS name

Otherwise you could try something like this:

SELECT  age, name  FROM    users  UNION ALL  SELECT  TOP 1 25 AS age, 'Betty' AS name  FROM users  WHERE (SELECT COUNT(*) FROM users) >= 2

Note that i've used UNION ALL since it doesn't seem that you want to eliminate duplicates.

Played around here: http://sqlfiddle.com/#!6/a7540/2323/0

Edit: Instead of my second approach i prefer Zohar's. So if you can use If....Else prefer that otherwise WHERE (SELECT COUNT(*) FROM users) > 1 without a table.

Something like the following should work:

SELECT  age, nameFROM    usersUNION ALLSELECT age, nameFROM (SELECT  25 AS age, 'Betty' AS name) xCROSS APPLY (SELECT COUNT(*) FROM users) y(cnt)WHERE y.cnt >= 2

Second part of UNION ALL will be NULL in case users table has less than 2 records.