Monge was 49 years old when Fourier attended his lectures:-

Monge has a loud voice and he is energetic, ingenious and very learned. It is well known that his talent is particularly for geometry, physics and chemistry. The subject that he teaches is a fascinating one, and he describes it with the greatest possible clarity. He is even considered to be too clear, or, rather to deal with his material too slowly. He gives individual practical lessons to his students. He speaks colloquially, and for the most part precisely. He is not only to be considered for his great knowledge but is also greatly respected in public and in private. His appearance is very ordinary ...

Lagrange was 59 years old when Fourier attended his lectures. This is interesting given the first sentence of Fourier's description:-

Lagrange, the foremost scholar of Europe, appears to be between 50 and 60 years old, though he is in fact younger; he has a strong Italian accent and pronounces an 's' as if it were a 'z'; he dresses very quietly, in black or brown; he speaks colloquially and with some difficulty, with the hesitant simplicity of a child. Everyone knows that he is an extraordinary man, but one needs to have seen him to recognise him as a great one. He speaks only in discussion, and some of what he says excites ridicule. The other day he said "There are a lot of important things to be said on this subject, but I shall not say them". The students are incapable of appreciating his genius, but the teachers make up for that ...