You're asking two questions.
If you're talking specifically about youthful muscular gains as a predictor of later-in-life muscularity, I suspect it's false from a "muscle memory" standpoint. Your body creates new muscle cells and eats them up with equal aplomb.
I suspect, however, there's a correlation with youthful muscle gains and late-in-life muscularity (but not a causal relationship). It is likely due to establishing a habit with its own (psychological) reward feedback. The earlier the habit forms, the more enforcement the habit enjoys.
If you're talking about general health, as in likelihood of resisting illness and death, youth health again has a correlation to late-in-life health, but not a completely causal relationship; many late-in-life illnesses are not lifestyle-dependent (obesity, Type II diabetes) but are inheritance-dependent (cancer, Alzheimer's).
Snapshot views of customers as a predictor of behavior/risk are used by insurance companies, and increase in accuracy with the addition of more apt variables. However, individual snapshots often cancel each other out, particularly if enough time has passed.