A stupid argument if you ask me.
1. China has close to 1.4 billion people! That's 1,400,000,000. Taiwan has 23.57 million (23,570,000).
It's going to take a lot for that demographic bubble to actually impact military efficacy.
In any case such a war is largely technological in nature and increasingly reliant on autonomous systems.
Note also Chinese military started massive reductions in numbers once it started modernising. These started in 1985 when the military was cut by 1 million people. You don't need masses of badly supported infantry in a modern war.
-----
2. Taiwan too has an ageing population with population growth stagnating.
Taiwan in fact has an older population than China.
Median ages:
China: 38.4 years
Taiwan: 42.5 years
Japan: 48.4 years
South Korea: 43.7 years
USA: 38.1 years
Australia: 37.2 years
EU: 43.7 years
India: 26.8 years
Sub-Saharan Africa: 19.7 years
----
3. One child policy saved China, Before that population growth was unsustainable (hence 1.4 billion people today).
Curbing population growth was one of the key things the Chinese did to stabilise the country and make it suitable for economic growth.
4. The gender imbalances incurred because Chinese traditionally prefer male children. This resulted in massive abortions and even infanticide of female children. (Another example of how culture is not just the food you eat and the clothes you wear which is what most idiot westerners think it is).
----
5. Rampant population growth is not good for economic development as GDP growth is chewed up by high population growth. IE Pie grows but it's shared amongst more people.
This happens in Australia a bit. Government etc bragged about near 3 decades of positive GDP growth but neglected to mention 3 GDP per capita recessions whereby the share per person shrunk thanks to extremely high immigration.
Indeed rampant population growth puts strains on everything from education to health to infrastructure.
----
6. China exports large chunks of people. The government supports this as a way of expanding Chinese influence at all levels. Note this also helps with the "restless males" or whatever idiot concept these guys harp on about as vast majority of emigres are males and often shunted around to work on Chinese projects around the world.
Eg. Croatia is building a large new bridge and the work was given to a Chinese firm (EU funded). All the workers are Chinese and they're housed separately on cruise ship and in barracks.