The structure mentioned in the previous answer is a good approach for most of the cities/countries. However, for hot tourist destinations like Dubai, one needs to consider the number of tourists that visit that particular city/country in a year.

On average, Dubai gets 12 million tourists (nearly four times the population of Dubai). If you use the similar approach and add that number to the one mentioned in the other answer, you arrive at the approximate number. Feel free to change assumptions regarding age distribution and life expectancy of tourists (Wouldn't change by more than 10-15% anyway).

(editiert)

For this, I would lay the structure and assumption down as follows

# of people aged between 0-20 = (Total Population of Dubai / Avg. life span of the local population or the world) * # of years (0-20)

Key assumption, people distribution of ages are even across the board. Stating, that there are the same number of people at every 1 year.

Hence,

# of people aged between 0-20 = ( (assumed) 3.5M / (assumed) 75 ) * 20

= 0.93M

Note, for this market sizing you should be aware of what the population of Dubai is, or approximate of it. Also 75-80 years is the average lifespan globally, this can vary by region.