instead of using mulriple if's like:

if @user = ''
 begin
  if @age = ''
   begin
    select * from customers
   end
  else
   begin
    select * from customers where age = @age
   end
 end
else
 begin
  if @age = ''
   begin
    select * from customers where [name] = @user
   end
  else
   begin
    select * from customers where [name] = @user and age = @age
   end
 end

you can use the COALESCE() like:

if @user = ''
 set @user = null
if @age = ''
 set @age = null

select * from customers where [name] = coalesce(@user, name) and age = coalesce(@age, age)