-I am 16 years old, can I implant it anyway? (skin elasticity, veins structures or other things)
No problem. By the age of 16 your major bone growth is slowing down and things should stay pretty much in place as you finish growing.
-How many years can it last?
The materials of the xNT will last your lifetime. The chip inside has two factors that govern “life” - data retention and write cycle count.
Data retention is how long a memory block will retain the data that is written to it before the storage medium loses bits and bytes to fading electrical charges. If you write data to memory block 02, that memory block can hold that data reliably for 10 years. If you write new data (or “refresh” the same data) at the 9 year point, then the data will last another 10 years in that memory block. Writing data to a memory block “resets” data retention.
Write cycle count is how many times a memory block can be written to before it can no longer reliably store data. The NTAG216 chip in the xNT has a data retention of 10 years and a cycle count of 100k. So, if you wrote data to the xNT once per day, the chip would last 273.9726 years… or, if you wrote data to the chip once every 10 years (the end of the data retention period), the chip would last 10,000 years.
-A loss or weight gain can create problems ??
Not an issue, unless it’s severe… then it may be a health issue 
-As I said, I come from Italy, can you guarantee me a good packaging (shipping can be dangerous for sterile products)? (We do not have a good postal system 
)
Hmm, well the product comes in it’s own box, and that goes into a shipping box. It should arrive in good condition.