"I am nineteen years old."
because Japanese got 2 reading system for kanji which come from Chinese:
On'Yomi (音読み) and Kun'yomi (訓読み), for respectively Chinese and Japanese reading. in this case 歳 can be read as :
サイ - Sa i
セイ - Se i
とし - To shi
とせ - To se
よわい - Yo wa i
As for which one to use when, if there is a group of kanji (2 or more) it tend to be the On'Yomi reading, and for only one kanji it tend to be the Kun'yomi reading but like for everything in japanese there are exceptions so, yeah.
And duolingo tend to use a different audio file if the kanji/Kana is alone or in a "block of words", or used in a sentence.
Hope that was clear enough.
If you scroll down far enough this article explains it. https://www.japanesewithanime.com/2017/04/age-years-old-in-japanese.html?m=1
I think its because when you are made to select the kanji alone it only plays one audio pronunciation. The correct pronunciation is above, so if you every need to make double check the pronunciation just use the forum audio.
They're interchangeable in this case. Both are correct. The one Duo uses (the "right" one) is more complicated, and children aren't expected to know it. Whereas 才 is simpler and easy to remember.