Since these figures are not directly available via Google search, I calculated them using the mid-year population estimates from the World Population Prospects (WPP) 2024:
World Population: In 2024, the global population is estimated at 8.2 billion (8,162 million). The population is projected to peak in 2084 at 10.3 billion.
Children Under 5 Years Old: The number of children under 5 is 647 million in 2024. This figure peaked in 2017 at 698 million and is projected to decline to 550 million by 2100.
Children Aged 0–14 Years: The population of children aged 0–14 peaked in 2020 and stands at 2.0 billion in 2024.
Children and Adolescents Aged 0–19 Years: The population of those aged 0–19 years is 2.7 billion (2,668 million) in 2024, which marks the peak year for this age group.
Population Under 25 Years Old (0–24): The number of people aged 0–24 years is 3.3 billion in 2024. This population is expected to experience a slight increase and peak in 2030.
Also by sex:
World population
Source: World Population Prospects (WPP) 2024. Population on 01 July, by 5-year age groups.