The rule about numbers at the beginning of a sentence is and always has been about one- or two-digit numbers. The "reason" for the rule is so that these small numbers do not get "lost in the shuffle" of the words.
...I was driving north, I believe. There was a string of buses in front of me. There were too many to count. Twenty would be my guess.
Written in figures, the "twenty" might get lost in all the words.
The rule never intended these types of numbers to be in words.
...2013 was a profitable year for us.
...3,500 were sent in error.
Happy punctuating!
Margie