When nylon is fresh out of the mould it will be naturally brittle as it is in a dry state. However Nylon absorbs typically 2-3% moisture from the atmosphere within days. Some manufacturers will immerse items in water to temporarily absorb 6-8% moisture content to accelerate the process. The moisture content improves the strength of nylon, but this can not be artificially improved, unless you choose to run your car in deep puddles!
Adding heat to plastics and letting it cool slowly (aka annealing, e.g. boiling) will stress relieve the plastic but there will be a risk of dimensional instability. Therefore be careful as your part may not fit when finished.
http://www.annealingplastics.com/
Ageing of plastics is irreversible. There may be a temporary effect when boiling due to low conductivity and therefore whilst the part retains the heat it will be softer/less brittle in the short term.