Using the fact that Mexico has half as many people over age 65 compared to the US (in proportion to their general populations), we should expect about 2 out of 10 flu-related deaths in Mexico will be someone under age 65 (compared to 1 out of 10 in the US), even if the virus is no more dangerous for younger people than other forms of influenza.
According to a recent article on CNN, 9 out of 10 flu-related deaths in the US are among people older than 65. According to the CIA World Factbook, 12.8% of the US population is 65 or older, while only 6.2% of the Mexican population is 65 or older. Multiplying the US population of 307 million by 0.128 gives 39.3 million people in the US over 65. Of the 36,000 deaths in the US related to flu each year, 90% are over the age of 65, which is 32,400 deaths. Thus the rate for flu-related deaths of people over 65 is roughly 32,400 deaths/39.3 million = 824 deaths per million people. The same calculation for people under 65 gives 3,600 deaths/267.7 million = 13.5 deaths per million.
If we assume that these death rates are roughly the same in Mexico (they won't be, but without further data it's the best approximation I have available) we can estimate the expected number of deaths per month related to influenza in Mexico. Of the 111 Million people in Mexico, 6.2% are 65 or over, which comes to about 6.8 million people. Multiplying by 824 deaths per million for this age group gives about 5670 deaths per year. For the other 104 million Mexicans under age 65 we multiply by 13.5 deaths per million, to obtain 1405 deaths. The total for a year is then 7075. Dividing by 12 gives approximately 600 deaths per month. Since Mexico City has 20% of the population of Mexico, we guess that 20% of these deaths will be in Mexico City, which comes to about 120 deaths per month.
This may well be an under-estimate, both because the death rate for older people in Mexico might be expected to be higher than that in the US, and because the infection and death rates in a large city might be expected to be higher than in the rest of the country. But without data to make the right adjustments, it's the best I can do. By order of magnitude this estimate is not far off from yesterday's estimate of 200 deaths per month.
What is more interesting is the proportion of deaths by age. This outbreak of Swine flu is type H1N1, as was the Spanish flu of the 1918 pandemic. (They are not necessarily the same virus, this is just a common characteristic they share.) The Spanish flu was reported to have killed a higher number of young people than other forms of influenza, and so there is concern that this newer virus might also be more dangerous than "ordinary" influenza.
Based on the estimates above, 1400 deaths will be people under age 65, out of about 7000 total deaths per year, which gives 1400/7000 = 0.20 which is 20%. So we would expect about 20% of deaths in Mexico related to influenza would be under age 65. This is twice the proportion in the US (1 out of 10 is 10%).
So if you were to look at the proportion of deaths under 65 in Mexico and compare it to the proportion in the US without accounting for the fact that there are fewer older people in Mexico (in proportion to the population as a whole), you might think that this flu virus is indeed more dangerous for younger people. But all of my estimates have been based on general data about influenza, not any particular strain.
More generally, the CIA World Factbook says that the median age of the Mexican population is 26.3 years, while the median age of the US population is 36.7 years. Overall, the population of Mexico is younger than the population of the US. So one would expect more deaths of younger people in Mexico compared to the US (in proportion to their general populations), just because more of the Mexican population is younger.
How could we tell if this strain of H1N1 is actually more dangerous for younger people than most forms of influenza? In Mexico the number of deaths of people under age 65 would have to be significantly more than 20% of the total, while in the US it would have to be noticeably more than 10% of the total. I don't have data for that, but I assume CDC and WHO do and will do these calculations correctly, taking into account the different age demographics in Mexico.