In which estate did the soldiers come prior to the French revolution?
In my book it is written "peasants had to serve in the army..." and "army was not open as I was not a noble by birth"
The enlisted soldiers in European armies before the French revolution were mostly third estate lower class. Most of the officers in most European armies before the French revolution were second estate upper class.
That is the general rule. Every society had its own twist and variation on the basic social classes. And every armed force had some variation in the class composition of the various ranks.
"peasants had to serve in the army..."
Was true for those peasants who volunteered or were chosen to serve.
"army was not open as I was not a noble by birth"
was also true for ambitious young men who considered careers as army officers but were barred from officer positions because of their low status.
So it is inaccurate to imply that privates in the army were mostly nobles before 1793. Privates were volunteers of low status or else were peasants conscripted against their will to fill enlistment quotas. Nobles or gentlemen in the enlisted ranks were rare in all pre revolution armies.
The two quotes relate to two different time periods.
Before the Revolution:
The medieval division of society into "those who fought (nobility), those who prayed (clergy), and those who worked (everyone else)" still held strong and warfare was considered a domain of the nobles.
Remember that, unlike England with its primogeniture, in France the title was inherited by all sons, there there were plenty of petty nobles with no means of self-support besides their sword.
After the Revolution: