In 1594, Galileo invented the sunction pump. He used it to draw water out of the ground like a syringe and was perplexed as to why the water would not rise past a certain height (10m). Galileo was 30 years old when he did this
Otto von Guericke
in 1650, when Otto was 48 years old, he developed a pump that created a vacuum so strong it took two teams of eight horses each to pull the two metal hemispheres apart. He believed the hemispheres were held in place by atmospheric pressure rather than the vacuum
Evangelista Torricelli developed the first barometer in 1643. Once Galileo's secretary, he continued his mentor's work and determined the limit to the height of Galileo's water pump was due to atmospheric pressure. Evangelista was 35 years old when he invented the barometer
in 1646, at the age of 23, Pascal discovered that as altitude increases, air pressure decreases. He determined this by travelling up and down a mountain with a barometer
In 1661, at the age of 32, Huygens developed the manometer to measure the elastic force of gases
in 1811, at the age of 35, Avogadro suggested the the pressure in a container is directly proportional to the number of particles in that container.
In 1801, at the age of 25, Dalton stated that the total pressure exerted by the mixture of non-reactive gases is equal to the sum of the partial pressures of individual gases.
Joseph Louis Gay-Lussac
In 1808, at the age of 24, Gay-Lussac observed the law of combing volumes. He noticed that, for example, two volumes of hydrogen combined with one volume of oxygen to form two volumes of water.