I don't know

Thank you very much for your honest answer. You are an expert, a neurologist.

So I conclude boldly but probably correctly that nobody knows

what the reason is why hereditary spastic paraplegia (HSP) starts at a particular age.

I still feel that my question is quite important, but obviously not the right question.

Now my my new question is:

what to ask next to really make some kind of progress?

Hereditary spastic paraplegia is a heterogeneous group of disorders, the predominant signs and symptoms of which are lower extremity weakness and spasticity. Defects in various pathways (e.g. axon transport, mitochondrial function, myelin formation, ER-stress response, endosomal trafficking, Ubiquitination, etc) have been shown to cause this disorder. However, all HSP subtypes are characterized by corticospinal tract axonal degeneration.

Regarding the age of onset, there is indeed a wide variability between different subtypes and even in one subtype within a family. For instance, a huge variation in the age of onset (6–60 years) has been reported in an Italian family with a mutation in Atlastin (S259F) [Smith, Bradley N., et al. "Four novel SPG3A/atlastin mutations identified in autosomal dominant hereditary spastic paraplegia kindreds with intra‐familial variability in age of onset and complex phenotype." Clinical genetics 75.5 (2009): 485-489].

We certainly need large scale genomic studies in order to make some progress in understanding the underlying pathological mechanisms involved.