While birthday milestones are a significant part of modern culture, there is no specific or traditional gift that is given for a 90th birthday.
However, just because there is not a traditional gift that should be given, does not mean a gift cannot be special. According to Birthday Gifts and Celebrations, gifts that hold special meaning make wonderful 90th birthday gifts. For example, gifts that represent the youth or childhood of the birthday person are ideal. Other gifts for a 90th birthday can include simple tokens of appreciation for that person such as cooking that person's favorite meal or even inviting family and friends over for a small celebration.