Curtis is 23 in the story and is known by the nickname, ‘Ceps, for his muscular body.  One regret Curtis has is that when he was younger, he treated a girl bad and now realizes he cares about her.  Her name is Lateesha, and she just moved back from her aunt’s home in Michigan. Curtis knows that Lateesha loves tomatoes, so he sets out to win her back by planting tomato seeds in the garden. Curtis has a green thumb, and his tomatoes start to get the attention of the other gardeners. When someone starts to steal his tomatoes, he “hires” a young, homeless man to sleep by and guard his plants. 

Curtis wins back Lateesha’s heart when he puts up a sign that reads, “Lateesha’s Tomatoes” in the garden. 

For Curtis, the garden is a way to prove that he has changed and is a more sensitive, caring man.  Because he raises tomatoes, something Lateesha loves, he hopes that it will show her how much he cares about her.  Therefore, the garden is a symbol of how anyone can change, grow, and redeem himself if given a chance.

To recap and answer your questions-- 

State mentioned—Michigan

Curtis’ age—23

Garden is a symbol of redemption.

I hope this answers your third question correctly as I wasn't sure what you meant by "the effect of the association for Curtis".  I'm reading it as how the association with the garden affected Curtis and what it means for him.