This is not exact ur requirement:
Execute once...
declare @t tABLE( id int, users varchar(10), age int)
insert into @t
SELECT 1, 'mohan', 40 union all
SELECT 2, 'raju' ,null union all
SELECT 3, 'kalli', 43 union all
SELECT 4, 'asha', null union all
SELECT 5, 'jisa', 43 union all
SELECT 6, 'jaju', null union all
SELECT 7, 'jazxdfju', 45 union all
SELECT 8, 'jaju', null union all
SELECT 9, 'kagvewlli', 43 union all
SELECT 10, 'abxvsha', null union all
SELECT 11, 'vgsfsa', 40
;with cte AS (SELECT distinct id, age FROM @t WHERE age is NOT NULL)
UPDATE @t
SET age = c.age
FROM @t t1
cross apply (SELECT top 1 age FROM cte WHERE id >= t1.id ) c
where t1.age is null
SELECT * FROM @t
--
Chandu