It is not easy to cook and mash green beans. But it is so nutritious it will worth your efforts. According to some doctors, babies may start solids on month 4. If your doctor also believes this you may start adding green beans to your babies food after month 4. But since it is hard to mash it, you may prefer to feed green beans to your baby at month 6-8. 
Keep in mind 
Green beans can be kept in the fridge for maximum 5 days. You should keep it inside a freezer bag. But don't wash it before you put it in the fridge. Wash it when you will eat it. 
If you wish to boil and cook your green beans, first boil water in your pan. Then add the green beans. Avoid putting too much water. Your green beans will be done in approximately 15 minutes. 

You can boil green beans will and mash it or you may try the other green bean dish alternatives for older gourmets like green beans with meat and green beans salad. If you ask "We already know these, what else can we cook with green beans?" here are the original green bean recipes of Gurme Bebek, please click.
Nutritional value per one cup of green beans (110 gram);  
Vitamin A - 759 IU 
Vitamin C - 17.9  mg 
Folate - 40.7 mcg 
Vitamin K - 15.8 mcg 
Calcium 40.7 mg 
Magnesium 27.5 mg 
Phosphor 41.8 mg 
Potassium 230 mg  
Omega 3- 39.6 mg 
Omega 6- 25.3 mg 
Sugar 1.5 g 
Fiber 3.7 g 
Protein 2 g 
Calorie 34