How old were Boaz and Ruth when they got married?

The precise age difference between Boaz and Ruth is unknown. The bible does offer some clues to determine that Boaz was a bit older than Ruth as seen here in verse 10, referring to Ruth not going after young men: 

Ruth 3:9 He said, “Who are you?” And she answered, “I am Ruth your maid. So spread your covering over your maid, for you are a close relative.”
Ruth 3:10 Then he said, “May you be blessed of the Lord, my daughter. You have shown your last kindness to be better than the first by not going after young men, whether poor or rich.

In addition to this, we know that Boaz was a relative of Naomi’s deceased husband:

Ruth 2:1 Now Naomi had a kinsman of her husband, a man of great wealth, of the family of Elimelech, whose name was Boaz. 

Naomi and her husband were considerably older than Ruth so we could draw a conclusion that Boaz was closer to the age of Naomi than Ruth, however, this does not conclude a specific age difference. 

Scripture quotations taken from the (NASB®) New American Standard Bible®, Copyright © 1995, 2020 by The Lockman Foundation. Used by permission. All rights reserved. www.lockman.org