Complex sentence: At the age of 15 she started writing for Canadian newspapers and magazines, and went on to contribute to the "Woodstock Daily Express", from 1890 to 1909, under the pseudonym "Heather".
Split into simpler sentences: At the age of 15 she started writing for Canadian newspapers and magazines. From 1890 to 1909 Mackay contribute to the "Woodstock Daily Express" using the pseudonym "Heather".