Looks like G L. The L may be an S, but, on line 12, the "S" in Sarah looks to be distinct from her middle initial; in Line 1 same page the "L" in Lucinda appears to be the same as G L's middle initial.
EDIT: Also, the "G" appears to be the same as a "Garner" near the bottom of that page, and "Gill" on the preceding page.
EDIT 2: I misread the question. Read it as he is ON line 15, rather than AGE 15. My apologies.