Acell = {A1,A2,A3,A4,....};
maxL = max(cellfun(@length, Acell))
M = zeros(maxL,1);
for n=1:maxL
    M(n) = mean(cellfun(@(V) V(n), Acell((cellfun(@length, Acell))>=n)))
end

Acell = {A1,A2,A3,A4,....};

maxL = max(cellfun(@length, Acell))

cellfun(@(V) V(n), Acell)

Acell((cellfun(@length, Acell))>=n)

cellfun(@(V) V(n), Acell((cellfun(@length, Acell))>=n))