One space or two at the end of a sentence?

One.

APA Publication Manual 6th edition: Spacing (4.01). Using two spaces after periods ending sentences aids readers of manuscript drafts.

Nothing Says Over 40 Like Two Spaces after a Period!
Unless you are typing on an actual typewriter, you no longer have to put two spaces after a period.

For age, use numerals and hyphenate

According to the Chicago Manual of Style (and other style guides), you always use numerals for ages, and hyphenate like this: 23-year-old.

APA page margins

Set uniform margins of at least 1 inch (2.54 cm) on the top, bottom, left, and right of every page. Use your word-processing software to add a header that will appear at the top of every page that includes the running head and the page number. The header appears within the top margin, not below it.

From the APA Publication Manual 6th edition (section 8.03, pp. 229-230).

For more information about editing and

formatting of essays, manuscripts, and theses,

see the Owl Editing website.