It has been stated by Oda that One piece Movie 10 will be Luffy’s last adventure as a 17 year old meaning that it’s likely we’ll either see Luffy turn 18 or a time skip.

Possibilities:

Luffy turns 18, this basically means he will have his first birthday on screen and turn 18.

A timeskip of 1-5 years.

Something will happen to him during the Marineford arc/Whitebeard war saga, and he will need to recover for a few weeks or maye even years or something.

Another theory is that the timeskip will end when he meets up with his nakama again, because at this moment they are scattered all over the world, so it might take a while until they reach eachother again.

Any other suggestions are welcome!!