My mother's lilac scent doesn't smell the same

IFRA's explainer on its standards says a material can be prohibited, restricted to a maximum quantity or given a purity specification, and that the standards are normally amended every three years. So a formula can definitely have to change between a childhood and now [lilac or not].

My mother's rose and powder went the same way. I thought my nose had gone.

Could be both. The Class on why perfumes disappear and how the Osmothèque keeps them lists four reasons, and an old bottle ageing badly is one of them.

Memory plays its part too. Whatever you smell now is up against forty years of missing her.

That's kind, and probably true. I'll take the four reasons as my answer and keep the lilac in the garden.