Does the holiness of a person grow over time? Does the holiness of a person grow over time?
If not, does the acts of charity/love of a person made in their youth matter if they lose their faith later in life?
The only way to get to heaven is based on how much of a supernatural love we have in us.

(CCC2000) Sanctifying grace is an habitual gift, a stable and
supernatural disposition that perfects the soul itself to enable it to
live with God, to act by his love.

Does this love fluctuate when we gain or lose it? Or is it chronological in the sense that there is always some element of it in us when we gain it?
I'd like to know the catholic view.
Holiness is how godlike one is. God is charity (1 Jn. 4:8). Since charity can increase (Summa Theologica II-II q. 29 a. 4) or decrease (a. 10),* so can holiness increase or decrease.
*cf. On Love and Charity: Readings from the Commentary [of St. Thomas Aquinas] on the Sentences of Peter Lombard pp. 31-55, 66-80