this is a very broad question, because intelligence has different types and different definitions. to keep the answer wide, let’s simply define and discuss intelligence as the brain’s ability to process information.
the ability of the brain to process information depends on the number of connections between neurons, their functionality, and the amount/composition of proteins involved in these processes. both genetic factors and environmental factors determine them.
the number of synapses in the brain, its functionality, and protein composition are not stable and vary according to activity (see synaptic plasticity). so the more you use your brain for intellectual activities, the more your brain is suited to these functions. however, the brain’s ability to adapt to it decreases with age. so, especially in the early years of life, the more active you use your brain, the smarter you are. children’s language learning skills for example.
studies in experimental animals show that learning processes increase the total number of synapses during the youth. learning processes in the following periods have an effect on the size of synapses beyond the number of synapses. there are also silent synapses in the brains of young animals, but as the name implies, there are silent synapses at that time (see silent synapses). these synapses are rapidly activated along with learning processes and are responsible for increasing learning skills at a young age.
the importance of genetic factors is of course very large, but it is excluded from the scope of this article because it is not possible to change them directly.
in short, run your brain at high speed and trust synaptic plastic.