What are some consequences of buying an older HDB flat when I'm 44 years old?

If the old flat remaining lease can't cover u until 95 years old, the cpf usage will be pro rated.
So next time when u want to sell, the buyer will face similar problem and it may affect the resale value.
If u intend to keep for the rest of yr life, then no big issue. Otherwise u may want to avoid those flat that alr more than 40 years old.