DHI hair transplantation to lower the hairline at the age of 27? (Photos)

Number of grafts depend on the area that needs to be covered. Before you lower your hairline, keep in mind that how it will look in next 10-15 years. With the age hairline recedes, so lower your hairline enough where it will look good for now and later in future. If you lower the hairline too low, it might look good now but will not look natural or decent according to the age later.

There is no age limit for hair transplantation. As long as you have a healthy donor area, you can undergo hair transplantation at any age. When it comes to hair transplantation, there are two major techniques FUE and FUT. By looking at the picture provided by you, FUE or follicle unit extraction seems like the right procedure.
FUE is also one of the best ways to reduce the forehead. In your case, you might need around 18000 to 20000 grafts to get a good result and you should be able to get a denser forehead with just a single session of hair transplantation. Hence, the diagnosis made by your doctor is quite good and you can go ahead with the procedure planned by the surgeon.
Coming back to the DHI part of the treatment, DHI is just a method of implanting the draft into the donor area. There are other ways of implanting the hair such as ice graft technique, sapphire blade technique, golden punch technique, and organic hair transplantation. In the real world, there is no superior technique amongst these and, it only depends on the surgeon and his expertise.
Coming back to the type of hair transplantation, you can also look into the SmartGraft technique, which involves no incision or stitching. The patient will be given local anesthesia and the procedure will be done in no time. With this procedure, one can see the complete result in just 9 months after the treatment.