Mila Kunis & Ashton Kutcher - then and now

Yep! She told the producers that she was going to be turning 18, she just didn't say when. They later found out the truth, but thought she fit the role perfectly so they kept her.

it was not the 70s as in the decade, it is talking about that 70s show (a show in which both were in)

The show's name is That 70s Show, not that it was filmed in the 70s

If by nearing 18, you mean four years away from 18, then yes. She was nearing 18.

Maybe you should look it up again. She /was/ 14 when she started the show.