Cycle changing as you get older.

Stacey • Have three amazing kids, two boys and a girl and now pregnant with our 4th and final. Due July 15th. Also have donated my eggs and have one donor conceived child.
The last three months that I have been ttc and tracking my cycles I have noticed that they have gone from a 28 day cycle before to now a 24 day cycle. Is this because I'm older? Has this happened to anyone else?