I’ve used this company since they were known as Carhire3000; they are a consolidator and offer very good prices. But carefully check the terms and conditions of each hire company, and I’d recommend you take out additional protection insurance from some like insurance4carhire.com
In the link below (search page) is included the driver’s age.
As someone who normally hires cars abroad I would always look on the TripAdvisor forums for advice. Those for Portugal will know who to suggest and it's usually local firms that give the best service over internationally known companies. I don't know what their age limits are but don't touch Gold Car with a bargepole.