Hello and welcome to the community!
If you want to change the fields of a struct after it has been created, it must be declared mutable, i.e.,
mutable struct Person
age::Int64
Marks::Vector{Float64}
end
you can also set default values for the fields like so:
Base.@kwdef mutable struct Person
age::Int64 = 0
Marks::Vector{Float64} = Float64[]
end
after this you can create a vector of persons
persons = [Person() for _ in 1:2]
persons[1].age = 4;
persons[1].Marks = zeros(2,);
# Second Person
persons[2].age = 5;
persons[2].Marks = ones(2,);