No version of Pages offers PDF/A document export support because the Apple OS X PDF Quartz libraries are still restricted to PDF v1.3 output. PDF/A-1 requires PDF v1.4, among its other criteria. You will need a third-party, word processing application that uses its own PDF generation package that includes the ability to produce PDF/A output. LibreOffice v5.1.0.3 (current as of this post) has that checkbox in its Print panel.
I exported a Pages document to Word .docx, and then opened that in LibreOffice Writer, where I could export as PDF/A-1a.
Here is the Export to PDF panel in LibreOffice Writer:
And the PDF/A notification when opening this PDF in Adobe Reader 11.0.15.