59 and still feel younger

i’m 59 years old i have been working out steady now for about two months. i would like to know what i need to do to build muscle and what i maybe doing wrong. here is my work .monday chest tri shoulders,tues back bis legs off wednesday.then thurs chest tri shoulders fri back bis legs no abs not worried about them.I do four sets 12 reps per set.last time i worked out steady was about three months ago before my knee surgery that is why i don’t do squats.Now for the excersices db bench press .flat bench fly,bb bench press,seated french press,reverse tri push down ,stright bar push down.db soulder press, front lat raise, reverse flys.close grip pulldown,lying pulldown,stright arm pulldown.standing db curl,seated db curl,lying db curl,leg press ,leg extention ,standing leg curl thank you

You are not following a structured periodized plan. Here are a bunch from my good friend Dr. Jim Stoppani. http://www.jimstoppani.com/home/featured_workouts