Depends on what we are looking at.
Talking official jewels the George IV diadem dates back to 1820. Both the Imperial state crown and St Edward's crown have been revised and the current versions only date back to the 20th century.
(from what I can find, I may have missed something older)
Tiaras:there only a handful that are not from the 21st century.
-Queen Adelaide's fringe tiara dates back to 1831.
-Oriental circlet was designed by Prince Albert in 1853.
-The teck hoop tiara is said to date back to 1860's (crescent may as well)
The modern sapphire was acquired by the queen in the 60s and converted to its current tiara status. But it did originate in Belgium with Princess Louise of Belgium and dates back to the late 1800s.
earrings:
-the corronation earrings date back to Queen Victoria in 1858
necklaces (we get older here)
-The queen Anne and Queen Caroline pearl necklaces- dont have exact dates but Caroline wore hers to her coronation in 1727. Anne's would be even older as she died in 1714. The current queen was given them as a wedding gift from her parents.
The Queen Anne and Queen Caroline Pearl Necklaces | The Court Jeweller
brooch: here we have some more dating back to at least 1800s
-The Prince Albert brooch was made for Victoria before their wedding in 1840
-the Victoria bow brooches date back to 1858
-Duchess of Cambridge brooch dates back to the 1850s
bracelets:
-Queen Victoria's bracelet dates to 1838
From Her Majesty's Jewel Vault: Queen Victoria's Bracelet
-the rose of york bracelet: gift to Queen Mary on her wedding day in 1893