User blog comment:Omega natsu2/...One Year Has Gone By/@comment-121.54.54.50-20150127231443/@comment-7418318-20150130095428

Depends on how you go about their age.

From the start of the series to the start of the GMG only six month passed for those that acompanied Lucy to the Celestial Spirit World and nine month for the other people who were on Tenrou.

That means that physically Lucy's still 17.5 (plus however long it was until the Tartarus arc).

Now you could go by date alone, but then you'd also have to count the time spent in Fairy Sphere.

To say she already was 18 before the second timeskip would be a mix of both methods by ignoring the forzen years, but counting the frozen month.