1)
The previous two verses said "Ko Amar Hashem." Why is this repeated again?
1.
Radak: It says so about every consolation, to strengthen the consolation - the good will certainly be.
2)
Why does it mention old people in the streets?
1.
Malbim: This comes to remove the second reason why they feared - lest Daryavesh exile them again. Here it promises that the Shalom will be so great that even the weak people - the old and children (5) will be in the streets without fear.
3)
Why does it say "mi'Rov Yamim"?
1.
Rashi: He needs a walking stick due to old age.
