It was real estate agent Ruth Reffkin’s birthday on Wednesday. The active Manhattan agent turned 71. And rather than thinking about retirement to spend time with her grandchildren — whom she adores — she started a brand new job at her son’s company, Compass, a tech-driven real estate brokerage. Why not when Robert Reffkin first set up Compass in 2013? “We had a conversation when the company was still small when Robert was starting, but I wanted it to be his thing and for him not to have his mother around,” she said wisely. But three years on, he and his team have built a business with 800 plus agents and 200 staff in cities including New York, Boston, Los Angeles, Miami, Washington, D.C. — and in the fall, his birthplace, the San Francisco Bay Area. Ruth, an agent at Ruten…