Last_Value Function return last value in an ordered set of values in SQL Server 2012. This function must have an OVER clause.
Syntax:
Last_Value(Column_Name) Over (Order By Column_Name ASC)
Example:

Create table #Temp
(
  ID  int identity(1,1) ,
  EmployeeName varchar(100),
  Age int
) 
Insert into #Temp(EmployeeName,Age)
Select 'Hitesh',25 union
Select 'Priyanka',23 union
Select 'Dev',33 union
Select 'Neha',25 union
Select 'Abhimanyu',22 union
Select 'Bhawesh',30

SQL Query using Last_Value Function

Select ID,
EmployeeName,
Age,
Last_Value(Age) OVER (ORDER BY Age ASC) as Last from #Temp

Result:
Last
22
23
25
25
30
33
Last_Value changes for each record and is equal to the current value in the result set.
To get same Last Value for All Records

select ID,
EmployeeName,
Age,
LAST_VALUE(Age) OVER (ORDER BY Age ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) as Last
from #Temp 

This will return 33 as the last value.