When the narrator says that Shelia is "out of reach" what does he mean?

In "The Bass, the River, and Sheila Mant," the narrator is a young fourteen-year-old boy that develops a crush on Sheila Mant. Sheila's family is spending the summer in a cottage at the same lake as the narrator. As Sheila sunbathes, the narrator learns all of her moods by watching her body language and expressions. Other suitors seem to be aware of her moods as well, knowing just the right time to approach her.
The narrator describes Sheila as the "middle daughter—at seventeen, all but out of reach." What he means by this comment is that because Sheila is a beautiful, older girl, he feels that he doesn't stand much of a chance with her. He shares that he has many "false starts" on his way to her house, often becoming too nervous to complete the journey. He eventually works up the courage to ask her out.