Exercise has no age limits!
Just because you are a middle-aged individual, doesn't mean it's too late take control of your health and improve the way you look.
A 2009 study compared the effects of a 8 week split body resistance training program on two groups of males aged 18-22 years and 35-50 years. Both groups gained similar amounts of muscle and strength (See the study here).
Other research shows that even individuals who are older than 60 years old can significantly enhance muscle size (See the study here). Next to increasing muscle growth, resistance training can also be an excellent way to treat sarcopenia (loss of muscle quality) and prevent osteoporosis (increased bone weakness) in older adults.
So, your age is not a good excuse to "give up" on training and a healthy balanced diet.