Translation:I am thirteen years old.
Duo needs to give a slow play option for words like these, or have lessons specifically on alternate pronunciations. I can't tell for certain how 十三 is supposed to be pronounced here. Same with some of the other numbers.
jū san is how you're supposed to pronounce it... but you're right, it's too fast
Here there's no change in pronunciation from the components. Though I'm not sure how it's different
I thought this too, but as you drill more phrases your brain picks it up super fast. Reviewing earlier content it feels almost impossible not to recognize most words and phrases. I used to pull my hair out trying to understand "good night" and "thank you for the meal" lol
I answered "He's 13" and got it right even if the answer seems to be "I'm 13". How do you tell who you're talking about ?
If you get asked someone's age, your answer inherits its subject from the previous sentence (in this case person being asked about).
If you need to specify it's, say, your own age, you'd add a subject to your sentence. E.g. 私は二十五です for "I am 25."