One. Single. Year.

One is singular.  So why do some parents say "one years old" when referring to their baby's first year of life? 

If I have had an iPod for 365 days, I don't say I've had it for one years.  Just one year. It's a year old.  Same goes for babies when they're that age.  They're one year old.  When they've lived through two years, you can then say two years old when describing a baby's age.