You can move them from youth to Elders Quorum/Relief Society. But looking at the handbook, there doesn't seem to be any leeway in the definition of "Single Member" or YSA which is to be at least 18.
Correction: Reading the Handbook, I only find allowances to advance YW prior to age 18 (Handbook 2: 9.1.4). No such language seems to exist for Prospective Elders (Handbook 2: 8.8) or being ordained an Elder (Handbook 2: 7.6)
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