Complex sentence: Susan was born in 1928 and is 12 years old when she appears in "The Lion, the Witch and the Wardrobe", and by "The Last Battle" she is 21 years old, as the final novel takes place in 1949.
Split into simpler sentences: Susan was born in 1928 and is 12 years old when she appears in "The Lion, the Witch and the Wardrobe". By "The Last Battle", she is 21 years old, as the final novel takes place in 1949.