REVERE – Police are investigating two armed robberies in the Shirley Avenue neighborhood that occurred this week, including one in which the victim was a 71-year-old man.According to Police Capt. Michael Murphy, the elderly man reported being pinned against a wall at gunpoint at 115 Thornton St. at about 6:30 p.m. Sunday.The victim had just entered his building when two people confronted him, according to police. He was not injured, but was robbed of cash, credit cards and jewelry, police said.Another armed robbery occurred 24 hours later at 151 Walnut Ave. where two masked men with a gun allegedly threatened two teenagers, police said.