What happens when you eat old chicken actually depends on how old the chicken is and how bad it is.
Just because chicken is old doesn't mean it's spoiled or bad as long as it's been refrigerated or frozen.
If you've had the chicken and it was raw but it was frozen for 6 months to a year then the old chicken is likely still very good and safe to eat.
If the raw chicken when thawed out smells bad or is very slimy feeling or has a bad taste to it then it's bad and should be thrown out.
Raw chicken can last for around 1 year in the freezer and if you refrigerate the raw chicken it should last for around 3 days.
Cooked chicken can last for around 6 to 7 days in the refrigerator and if you freeze the cooked chicken it can last for around 6 months and even up to a year.
If the chicken smells bad, tastes bad or is very slimy or has changed in color then it's likely bad and should be thrown out.