I work for a luxury fashion curator. I won't say who, but we have collections from Alexander McQueen, Versace, Saint Laurent, Balenciaga, Stella McCartney etc, so I'd like to think my day job gives me a good knowledge of timeless pieces for a pretty big age range 
The phrase "buy cheap buy twice" is very true when buying key pieces to last. You are much better off investing in some really good quality basics that are a staple throughout seasons and time periods.
A tailored nude trench coat will always come back into fashion year on year. They go with so many different outfits. They also look very chic and expensive if you get the right one.
Invest in a few oversized shirts, again go with sharp tailoring. Don't go for patterns or prints. Prints are very seasonal and trend dependent. Choose neutral colours - they flatter all ages.
A nice pair of jeans is a good investment. Go with dark colours and a slim fit.
An oversized blazer is a great piece for spring and autumn. I bought a mens blazer for length which I then took to a tailor who made it the perfect shape for me. I also chose a blazer with a lovely striped lining so I can roll the sleeves up if I want and it still looks styled.
I wouldnt suggest ballet pumps as they don't wear very well. Go with a classic black loafer that will retain its shape.
A pair of nice, simple black straps heels will also age well. Chloe and Saint Laurent have some classic styles.
The best pieces to invest in for this are accessories. A pair of oversized sunglasses will always be in fashion. Chanel are really worth it. As are Gucci. You can have those decades and they will always look great.
Alexander McQueen genuinely produce the nicest, long lasting bags I've ever seen. The quality is unbeatable and the pieces are always classic in silhouette. You can buy a McQueen bag and pass it through generations if you take care of it.
I know lots of people don't agree with spending lots of money on clothes but you can make certain items last a lifetime.