I have just done three days in Rome with MIL and her friend, and my SIL. (70).
Despite them acting like toddlers (need a wee, i'm tired, are we there yet) we had an amazing time. We stayed in an airbnb very near the Spanish steps. Everything you want to see is walkable. We bought fast track passes for the Vatican museum and the Sistine chapel, so no queue (about £20 each) and yes its busy when you get in but still beautiful.
The weather was unreal, so hot!
We did an open top bus tour on the third day as the olds were knackered and I really enjoyed that but you don't need to, you can walk to everything. The Rome marathon was on on the sunday which was cool.
Every meal we had in random restaurants was fabulous. We got off the bus at the bus station and walked a couple of streets and fell into a fantastic restaurants loads of locals were eating in, at like queens and had wine for 60 euros, 4 of us.
Loved Spanish steps, Pantheon, Trevi Fountain, Piazza Navona, Vatican Museum, Sistine Chapel, walking over the river the bridges were awesome. Did most of the sites during the day and then again at night to see them all lit up.
It is a stunning beautiful city, I always felt safe, and we had an amazing few days, I hope you do too.