ORANGE code

AGE        NAME
----------------
10             A
10             B
10             C
11             D
11             E

10          A,B,C
11          D,E

 SELECT AGE, CAST(XML2CLOB(XMLAGG(XMLELEMENT(NAME "NM", strip(NAME)))) AS VARCHAR(3000))
    FROM TABLE
  GROUP BY AGE 

10           <NM>A</NM><NM>B</NM><NM>C</NM>
11           <NM>D</NM><NM>E</NM> 

 SELECT AGE,
    REPLACE(REPLACE(REPLACE( 
    CAST(XML2CLOB(XMLAGG(XMLELEMENT(NAME "NM", strip(NAME)))) AS VARCHAR(3000))
   , '</NM><NM>', ',') , '<NM>', '')  , '</NM>', '') 
    FROM TABLE
  GROUP BY AGE 

10           A,B,C
11           D,E 

SELECT AGE, LISTAGG(NAME, ',') WITHIN GROUP (ORDER BY NAME)
   FROM TABLE 
 GROUP BY AGE