I've researched and found no maximum age to rent a car in Germany.
What you will need to check for is whether or not the particular car rental company that you wish to rent from will allow you to take their car into Croatia.
When I research driving restrictions in Europe, I check with AutoEurope for their driving guides for specific countries where you would rent.
According to their guide, when you rent a car in germany, most car categories can be driven anywhere in Western Europe without restriction, So Italy would not be an issue. (there may be restrictions on luxury car categories travel in certain countries)
AutoEurope will allow travel into the following countries as long as arrangements are made in advance with them:
Belarus, Bosnia, Bulgaria, Czech Republic, Croatia, Estonia, Herzegovina, Hungary, Latvia, Lithuania, Macedonia, Montenegro, Poland, Romania, Russia, Serbia, Slovakia, Slovenia, Turkey, Ukraine and Yugoslavia.
No mention was made of Slovenski -- but these are general quidelines (with AutoEurope) with regards to your query about on the rent of a car in Germany and travel into Croatia.
I strongly recommended that you look into specific geographic restrictions and cross border driving policies for the car rental company that you will potentially rent from to determine travel into Croatia and Slovenski in particular.
International Car Rentals
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