# Truly I say to you

"I tell you the truth." This phrase adds emphasis to what Jesus says next.

# in the new age

"at the time when God makes all things new"

# the Son of Man

Jesus is speaking about himself. 

# sits on his glorious throne

"sits as king on his glorious throne" or "rules gloriously as king"

# sit upon twelve thrones

"sit as kings on 12 thrones" 

# the twelve tribes of Israel

"the people of the 12 tribes of Israel"