Jamie Lynn Spears is all grown up!

Britney's lil' sis, now 21, spent quality time at the pool in Tennessee with her 3-year-old daughter Maddie .. who turns four next month.

Spears, who is hoping to become a country star, recently opened up to Glamour magazine about becoming a young mom at 16.

She tells the mag:

    "I had to make a decision that I could sleep with every night. I did feel responsible for the young girls and the mothers who I probably confused and let down. I apologize for that. But I wasn't trying to glamorize teen pregnancy."

Click thru to see another poolside pic.