First of all check the objective - is that search in google, or search everywhere (i.e. Amazon, Facebook, Spotify)
Then I would use the following approach:
- Size of US population - 320 M ppl
- Split the population into several age groups: Kids - 10-20, millennials 20-30, adults 30-60, elderly 60-80.
- Assuming that the people are spread evenly and there are 4M people of each age - calculate the size of each group (.e.g. kids 40M)
- Multiply the size of the group by internet penetration (i.e. 100% for millennials and 65% for elderly)
- Assume the number of queries per each age group per month (Use yourself and your personal experience as a benchmark for millennials and your Grandmother for elderly people) and multiply it with the number of people
- Sum up the age groups