I used to believe, although I don't now, that growing and growing up are analogous, that both are inevitable and uncontrollable processes. Now it seems to me that growing up is governed by the will, that one can choose to become an adult, but only at given moments. These moments come along fairly infrequently-- during crises in relationships, for example, or when one has been given the chance to start afresh somewhere-- and one can ignore them or seize them.I think this is basically right, though I wouldn't put it in terms of "becoming an adult." I'd put it in terms of "becoming the sort of person one wants to be." The older I get, the clearer it becomes that the idea of adulthood is nonsense. We're all still muddling through, the same way we always have.
Hornby seems to treat crises and fresh starts as distinct opportunities for growth. I'm inclined to think they are two necessary conditions. Turmoil reveals the things we'd like to change, fresh starts gives us the opportunity. This is why break-ups are growth experiences for almost everyone: the crisis and the fresh start come bundled together. In those situations, we have a choice about whether or not to change in ways that make us more like the person we want to be.