I am not aware of any direct mention of Ravana's age in Valmiki Ramayana but we can roughly calculate it.
In Yuddha Kanda, Ravana himself says that he was cursed by Anaranya:
[Ravana:] "Rama the son of Dasaratha is the man I think of whom Anaranya born of Ikshvaku dynasty formerly spoke, saying: 'O, the worst of demons, the worst of your race and the wretched one! In my race will be born a man who will slay you in battle with your sons, ministers, army, horses and charioteer." ६-६०-८
This is confirmed in UTTARA KANDA too that Ravana killed king Anaranya (born of Ikshvaku dynasty) and Anaranya cursed Ravana just before dying.
Also, Ravana had encounter with king Mandhata (born of Ikshvaku dynasty) as well - Was there an encounter between Ravana and Ikshvaku king Māndhātā?
All of this confirms that there was many generations gap bw Ravana and Lord Rama:
- 1 Manu -> 2 Ikshvaaku -> 3 Kukshi -> 4 Vikukshi -> 5 Baana -> 6 Anaranya -> 7 Pruthu -> 8 Trishanku -> 9 Dhundumaara -> 10 Yuvanaashva -> 11 Mandhaata -> 12 Susandhi -> 13 Dhruvasandhi -> 14 Bharata -> 15 Asita -> 16 Sagara -> 17 Asamanja -> 18 Amshuman -> 19 Diliipa -> 20 Bhageeratha -> 21 Kakutstha -> 22 Raghu -> 23 Pravriddha (Kalmaashapaada) -> 24 Shankana -> 25 Sudarshana -> 26 Agnivarsna -> 27 Shiigraga -> 28 Maru -> 29 Prashushruka -> 30 Ambariisha -> 31 Nahusha -> 32 Yayaati -> 33 Naabhaaga -> 34 Aja -> 35 Dasharatha -> 36 Rama ~ Baala Kanda: Sarga 70
Few kings like Harischandra and Rohitashva seems to be missing from this list.
It's mentioned in Valmiki Raamayana that most of these kings ruled for many thousand years and some time must have went in tapsya before and after rule. For Dasharatha it's mentioned 60 thousand years from birth.
"That great king Sagara could not get at any resolve in getting Ganga to earth even after a long time, and on ruling kingdom for thirty-thousand years he went to heaven." Thus Vishvamitra continued his narration about the ancestors of Rama. [1-41-26]
On practising ascesis in ascetic-woods for thirty-two thousand years that highly renowned king Amshuman achieved heaven as he acquired only the wealth of practising the ascesis. [1-42-4]
That great-resplendent king Dileepa on his part performed numerous Vedic-rituals, and he ruled the kingdom for thirty thousand years - to the delight of each of his subjects, but could not find a way to fetch Ganga. [1-42-8]
Thousands of years have rolled by while Bhageeratha stood practising his severe ascesis, oh, dextrous Rama, and then the lord and master of all beings, namely god Brahma, is well pleased with that great-souled king's ascesis. [1-42-13b, 14]
[Dasharatha] "Sixty thousand years have passed from my birth, oh! Vishvamitra, and this Rama is engendered at this age, that too with tribulations, hence taking Rama with you will be inappropriate of you. [1-20-10b, 11a]
If you consider 30 generations and 50k years average age then only it's 15 million (30 * 50,000) and we are not sure of exact number of lineage and age of each lineage. So Ravana's age would be in million years as per these references.
