Our immune systems are often able to destroy germs and virus particles. About how many of them does it take to make a 70 year old healthy male sick ? Any ideas ?
To the best of my knowledge, we don't yet know the infectious dose of the SAR-COV-2 virus in humans. In theory, it only takes one viulent virus particle to start an infection. In practice, it's pretty hard to measure, Published estimates of infectious dose for any pathogen don't often account for differences in age or sex, per se, so it's hard to address that part of your question.
Most of the estimates of SARS-CoV-2 infectious dose I've seen are essentially experts handwaving based on previous coronavirus outbreaks. I'm not saying they're useless, but they aren't necessarily empirical. For reference, there's a perspective article here, with a section discussing infectious dose. They cite an estimate from the 2003 SARS outbreak of 280 particles to cause infection in 50% of the exposed population (ID50). They also point out that this is similar to the ID50 for other animal corona viruses and the common cold in humans.
There's no guarantee that SARS-CoV-2 will be the same, but it's likely to be in the same ballpark.
Imke Schröder, J Chem Health Saf. 2020 May 11 : acs.chas.0c00035. Accessed via PubMed Central, PMCID: PMC7216769