Possible Goof ?

I noticed that Edwina (the eldest) was 20 when the movie started that's fine. That would make her 35 at the end of the movie.

Teddy (the youngest) looked to be about 2 or 3. Fifteen years later he would be 17 or 18.

Alexis (the middle child) was fifteen years after the disaster was said to be 17. But the child in the beginning looked to be about 6 as was her 6th birthday party aboard The Titanic. 15 + 6 equals 21 NOT 17.

Am I wrong?

P.S: Look at the child playing a teenage Teddy it's the same person who played Anakin Skywalker in Star Wars 1, 2 and 3.